Stereoselective Addition of Organotitanium Reagents to Carbonyl Compounds

Synthesis of Highly Oxygenated Bicyclic Carbasugars. Remarkable Difference in the Reactivity of the d-gluco and d-xylo- Derived Trienes

2,3,4-Tri-O-benzyl-D-xylopyranose was used as a starting material in the preparation of the corresponding triene, which underwent smooth cyclization to a polyhydroxylated hydrindane, as a single diastereoisomer. The analogous triene prepared from D-glucose did not undergo any cyclization even under high pressure.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

A modular approach to prepare enantioenriched cyclobutanes: synthesis of (+)-rumphellaone A

A modular synthesis of enantioenriched polyfunctionalized cyclobutanes was developed that features an 8-aminoquinolinamide directed C-H arylation reaction. The C-H arylation products were derivatized through subsequent decarboxylative coupling processes. This synthetic strategy enabled a 9-step enantioselective total synthesis of the antiproliferative meroterpenoid (+)-rumphellaone A.

Titanocene(III)-promoted Barbier-type crotylation of carbonyl compounds

A mild, highly regio- and stereoselective method for the crotylation of aldehydes and ketones mediated/catalyzed by titanocene(III) is described. Optimized conditions permit the selective generation of γ-adducts in high yields together with high stereoselectivity, with a predominance of anti stereoisomers.

Iridium-catalyzed anti-diastereo- and enantioselective carbonyl (trimethylsilyl)allylation from the alcohol or aldehyde oxidation level

Using the ortho-cyclometalated pi-allyl iridium precatalyst (R)-I derived from [Ir(cod)Cl](2), 4-cyano-3-nitrobenzoic acid, (R)-SEGPHOS, and allyl acetate, enantioselective transfer hydrogenation of alpha-(trimethylsilyl)allyl acetate in the presence of aldehydes 2a-i mediated by 2-propanol delivers products of (trimethylsilyl)allylation 4a-i in good isolated yields and with exceptional levels of anti-diastereoselectivity and enantioselectivity (90-99% ee). In the absence of 2-propanol, but under otherwise identical reaction conditions, carbonyl (trimethylsilyl)allylation is achieved directly from the alcohol oxidation level to furnish an equivalent set of adducts 4a-i with roughly equivalent isolated yields and stereoselectivities. To evaluate the synthetic utility of the reaction products 4a-i, adduct 4g was converted to the 1,4-ene-diol 5g via dioxirane-mediated oxidative desilylation with allylic transposition, the allylic alcohol 6g via protodesilylation with allylic transposition, and the gamma-lactam 7g via chlorosulfonyl isocyanate-mediated cycloaddition.

Mitomycins syntheses: a recent update

Mitomycins are a class of very potent antibacterial and anti-cancer compounds having a broad activity against a range of tumours. They have been used in clinics since the 1960's, and the challenges represented by their total synthesis have challenged generations of chemists. Despite these chemical and medicinal features, these compounds, in racemic form, have succumbed to total synthesis only four times over the last 30 years.

Highly diastereoselective addition of allyltitanocenes to ketones

A route with less congestion: A practical method for the highly diastereoselective preparation of anti tertiary homoallylic alcohols has been developed. The reaction of allyltitanocenes, generated by the reductive titanation of various allylic substrates with a titanocene(II) species, with a variety of ketones produced the anti tertiary homoallylic alcohols in good diastereoselectivity, even when using sterically less congested ketones (see scheme; Cp: cyclopentadienyl; Piv: pivaloyl).The reaction of allyltitanocenes, generated by the reductive titanation of allylic sulfides or allylic alcohol derivatives with a titanocene(II) species, with phenyl and sterically hindered alkyl methyl ketones produced anti tertiary homoallylic alcohols with complete diastereoselectivity. Even when sterically less congested methyl ethyl ketone and methyl vinyl ketone were employed, the anti homoallylic alcohols were obtained with unprecedented high diastereoselectivity. The observed anti selectivity suggests that the reaction proceeds by the addition of primary (E)-sigma-allyltitanocenes to ketones through chairlike cyclic transition states.

A convenient allylsilane-N-acyliminium route toward indolizidine and quinolizidine alkaloids

This review relates all the results that we obtained in the field of the total synthesis of indolizidine and quinolizidine alkaloids using a strategy of the addition of an allylsilane on an N-acyliminium ion. In this paper, we describe the synthesis of racemic indolizidine 167B and chiral indolizidines: (-)-indolizidines 167B, 195B, 223AB, (+)-monomorine, (-)-(3R,5S,8aS)-3-butyl-5-propylindolizidine and (-)-dendroprimine. Next, we relate the synthesis that we have developed in the quinolizidines field: (±)-myrtine and epimyrtine, (±)-lasubines I and II and chiral quinolizidines: (+)-myrtine, (-)-epimyrtine, (-)-lasubines I and II and (+)-subcosine II.

Synthetic studies on heterocyclic natural products

This article reviews past and ongoing research in the author's laboratory directed toward the synthesis of natural products displaying an azaspirocyclic framework, or incorporating a medium-ring nitrogen heterocycle. New synthetic technologies were devised in order to address the synthetic problems posed by the target molecules. Thus, efforts in the area of azaspirocyclic substances have relied on an oxidative amidation of phenols promoted by iodobenzene diacetate, whereas access to medium-ring nitrogen heterocycles has been secured by means of a ring expansion sequence that relies on the fragmentation of an aziridine triggered by a homo-Brook transposition. Details of the development of these technologies are presented, together with applications to the total synthesis of FR-901483, TAN-1251C, cylindricines, and mitomycinoids.

Metal enolates ofα-CF3 ketones: theoretical guideline, direct generation, and synthetic use

The difficulty as well as the significance of the direct generation of metal enolates of alpha-CF(3) ketones cannot be easily understood by chemists unfamiliar with F. In sharp contrast to the sunny side of non-F, hydrocarbon chemistry, F chemistry has long been overshadowed. Metal enolates of carbonyl compounds are synthetically important in C-C bond-forming reactions. However, the metal enolates of fluorinated carbonyl compounds have been severely limited to alpha-F metal enolates. Alpha-CF(3) metal enolates have generally been recognized as unstable and difficult to prepare because of the facile beta-M-F elimination. However, we have developed a direct generation and synthetic application of alpha-CF(3) metal enolates. Therefore, the present results regarding the direct generation and synthetic use of metal enolates of alpha-CF(3) ketones might be recognized as a real breakthrough for the general use of metal enolates in F chemistry.

Stereoselective intermolecular carboazidation of chiral allylsilanes

[reaction: see text] Easily available chiral allylsilanes were used as substrate for carboazidation reactions. For the first time, a substantial control of the stereochemistry of the azidation of acyclic nonconjugated radicals was achieved.

The intriguing reactivity of functionalized beta-amino alcohols with glyoxal: application to a new expedient enantioselective synthesis of trans-6-alkylpipecolic acids

New beta-amino alcohols possessing a vinylsilane moiety were reacted with glyoxal to produce lactones that were transformed in three steps in enantiopure pipecolic acid derivatives. The key step was a totally diastereoselective ene-iminium cyclization, whose mechanism can be viewed as a direct cyclization of the vinylsilane moiety onto the iminium ion. The reactivity of two beta-amino alcohols having an allylsilane terminator was also examined. Their difference of reactivity toward glyoxal can be ascribed to the intervention of a carbocation, which presents a behavior that differs according to the position of the trimethylsilyl group.

Asymmetric syntheses of new functionalized beta-amino alcohols via diastereoselective addition of organometallic reagents onto oxazolidines

Diastereoselective reactions between (S)-phenylglycinol-derived oxazolidines and two unsaturated organolithium reagents afforded chiral beta-amino alcohols having vinyl and alkynylsilane moieties. When the same reactions were performed in the presence of titanium isopropoxide, a dramatic change of regioselectivity was observed, from the alpha- to the gamma-position, thus producing beta-amino alcohols with allyl or allenylsilane functions. A rationalization of the observed diastereoselectivity was suggested for each case.

A highly atom efficient, solvent promoted addition of tetraallylic, tetraallenic, and tetrapropargylic stannanes to carbonyl compounds

Tetraallylic, tetraallenic, and tetrapropargylic stannanes (0.25 equiv) react with aldehydes in methanol to provide unsaturated alcohols in good to excellent yields (56-99%). These reactions proceed exclusively with allylic rearrangement for tetra(2-butenyl)tin 2b and tetra(1,2-butadienyl)tin 16c and predominantly with allylic rearrangement for tetrapropadienyltin 16a and tetra(2-butynyl)tin 6e. Allylation reactions also proceeded smoothly with reactive ketones such as ethyl pyruvate (9a) and cyclohexanone (9b). The corresponding TFA-catalyzed reactions of dimethyl acetals 4d and 4e are regiospecific with allylic rearrangement.

Total synthesis of (+/-)-stemodinone via an efficient ring-exchange strategy

A total synthesis of (+/-)-stemodinone, a tetracyclic stemodane diterpene, from the known tricyclic methyl olefin 11 is described. The key steps involve an efficient ring-exchange reaction and palladium(0)-catalyzed lactone migration. The ring-exchange strategy for controlling the stereochemistry was based on an initial Diels-Alder reaction to form a new ring followed by cleavage of the original ring. Cleavage of the original ring of the Diels-Alder adduct 9 was achieved by an initial regio- and chemoselective Baeyer-Villiger oxidation followed by the Pd(0)-catalyzed lactone-migration reaction reported by us.

Total synthesis of (+/-)-scopadulin

The first total synthesis of (+/-)-scopadulin, an aphidicolane diterpene, is described. The core structure (A/B/C/D ring system) was constructed by an initial synthesis of the B/C/D ring system by our reported methods and a subsequent A ring cyclization by intramolecular aldol condensation. A highly stereoselective cyanation of the tetracyclic enone by Et2AlCN gave a trans-fused A/B ring system with a beta-cyanide at C-4. Stereoselective construction of a quaternary carbon at C-4 was achieved by alpha-alkylation of the cyano group and conversion of the sterically hindered cyano group to a methyl group via our novel reaction for conversion of primary aliphatic amines into alcohols. Finally, the total synthesis of (+/-)-scopadulin was accomplished by a highly chemo- and stereoselective methylation at C-16 and modification of the C-4 alpha-functionality. The stereoselectivity observed in the MeTi(O-i-Pr)3-mediated methylation for the generation of a tertiary axial alcohol at C-16 is extremely high.

The first total synthesis of (+/-)-scopadulin, an antiviral aphidicolane diterpene

[reaction: see text] The first total synthesis of (+/-)-scopadulin was accomplished by a stereoselective construction of a quaternary carbon at C-4, conversion of the hindered cyano group to a methyl group via our novel reaction for conversion of primary aliphatic amines into alcohols, and a highly chemo- and stereoselective methylation at C-16.

Use of Manganese(II)-Schiff Base Complexes for Carrying Polar Organometallics and Inorganic Ion Pairs

This report concerns the carrier properties of [Mn(acacen)]-derived compounds toward polar organometallics, inorganic ion pairs, and salts. Such properties are the consequence of Mn(II) behaving as a Lewis acid and the O&arcraise;O bite of the bidentate Schiff base ligand toward alkali cations. The starting compounds, which occur in a dimeric form, [Mn(acac-L-en)](2) [L' = CH(2)CH(2) (1); L" = C(6)H(10) (2); L"' = R,R-C(6)H(10) (3)] have been synthesized either via a metathesis reaction from MnCl(2) or using [Mn(3)Mes(6)]. The reaction of 1-3 with lithium organometallics allowed the isolation of [Mn(acac-L-en)(R)Li(DME)] [R = Me, L = L' (4); R = Ph, L = L' (5); R = Mes, L = L' (6); R = Me, L = L" (7); R = Me, L = L"' (8)] as metalated forms, where the alkyl or aryl group is sigma-bonded to Mn(II), while the lithium cation is anchored to the Schiff base ligand. The metalated forms 4-8 react with PhCHO to give the corresponding lithium alkoxide, which remains bound in its ion-pair form to the [Mn(acacen)] skeleton in [Mn(2)(acac-L'-en)(2)Li(2)(OCH(Ph)Me)(2)](n)() (9). The use of 8, which has a chiral bridge across two nitrogen atoms, did not lead to a significant asymmetric induction in the reaction with PhCHO, because of the long separation between the lithium cation and the stereogenic center. The metalated form 4 was able to transfer the methyl group to the nitrile function to give the corresponding lithium-imide which then remains bonded to [Mn(acacen)] as the ion pair in a dimeric structure, as revealed for [Mn(2)(acac-L'-en)(2)Li(2)(DME){N=C(Ph)Me}(2)](n)() (10). Their reaction with 1 appears to depend on the steric bulkiness of the alkyl group in NaOR, resulting in either monomeric adducts, i.e. in [Mn(acac-L'-en)(2,6-Bu(t)(2)C(6)H(3)O)Na(DME)(2)] (11.2DME), or polymeric structures, like in [Mn(acac-L'-en)Na(DME)(&mgr;-OEt)](n)() (13). All the dimeric units reported in this paper show a slight antiferromagnetic coupling between the two Mn(II) assisted by bridging alkoxo groups.