Construction of Quaternary Stereogenic Centers via [2 + 2] Cycloaddition Reactions. Synthesis of Homochiral 4,4-Disubstituted 2-Azetidinones and Imine Substituent Effects on beta-Lactam Formation

Azetidin-2-ones: structures of anti-mitotic compounds based on the 1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one core

A series of related substituted 1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-ones have been characterized: 3-(4-fluoro-phen-yl)-4-(4-meth-oxy-phen-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C25H24FNO5 (1), 3-(furan-2-yl)-4-(4-meth-oxy-phen-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C23H23NO6 (2), 4-(4-meth-oxyphen-yl)-3-(naphthalen-1-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C29H27NO5 (3), 3-(3,4-di-meth-oxy-phen-yl)-4-(4-meth-oxy-phen-yl)-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C27H29NO7 (4) and 4,4-bis-(4-meth-oxy-phen-yl)-3-phenyl-1-(3,4,5-tri-meth-oxy-phen-yl)azetidin-2-one, C32H31NO6 (5). All of the compounds are racemic. The lactam and 3,4,5-tri-meth-oxy-phenyl rings are approximately co-planar and the orientation of the lactam and the 4-meth-oxy-phenyl substituent is approximately orthogonal. The chiral centres, although eclipsed by geometry, have torsion angles ranging from -7.27 to 13.08° for the 3 position, and -8.69 to 13.76° for the 4 position of the β-lactam. The structures display intra-molecular C-H⋯O bonding between the 3,4,5-tri-meth-oxy-phenyl ring and the lactam ketone. Further C-H⋯O inter-actions are observed and form either an opposing meth-oxy 'buckle' to join two mol-ecules together or a cyclic dimer.

Evidence for an umpolung type of [2+2] cycloaddition of 2-carbamoyl ketenes

2-Carbamoyl ketenes react as nucleophiles with iminium salts and the process leads to the formation of alkylidene malonamides.

Nitrenium ion azaspirocyclization-spirodienone cleavage: a new synthetic strategy for the stereocontrolled preparation of highly substituted lactams and N-hydroxy lactams

[reaction: see text] Although 1,4-cyclohexadienes 2, obtained through the Birch reduction of arenes 1, have found widespread use as masked beta-oxo carbonyl synthons 3, the possibility that 2,5-cyclohexadienones 5 might also be employed to the same end has been overlooked despite their ready availability. As part of our ongoing investigation of the synthetic chemistry of nitrenium ions, we have developed a novel and efficient strategy for the stereoselective preparation of di- and trisubstituted azetidinone, pyrrolidinone, and piperidinone derivatives, which features the ozonolytic cleavage of azaspirocyclic 2,5-cyclohexadienones 12. For example, ozonolysis of spirodienone 12c in CH2Cl2 and reductive workup with dimethyl sulfide generated unstable beta-formyl ester 21, whereas cleavage in MeOH followed by reduction with thiourea led to hemiacetal 22. While both 21 and 22 partially decompose upon exposure to silica gel, they can be trapped in situ, with a variety of weakly basic nucleophiles, to usefully substituted products. The requisite spirodienone substrates are readily accessible through the nitrenium ion cyclization of alkyl omega-arylhydroxamates 10, which proceeds with moderate to high diastereoselectivity.

Highly Diastereoselective Cyanation of Methyl Ketimines Obtained from (R)-Glyceraldehyde

[reaction: see text] Addition of trimethylsilyl cyanide to ketimines derived from (R)-2,2-dimethyl-1,3-dioxolan-4-yl methyl ketone to generate a quaternary stereocenter has been achieved with high yields and excellent diastereoselectivity. The stereoselectivity was found to be temperature and solvent dependent. The beta-hydroxy-alpha-amino nitrile of syn configuration was the major compound in kinetically controlled reactions, whereas the anti stereoisomer was obtained in excess in thermodynamically controlled reactions. Double stereodifferentiation under kinetic control conditions was successful, and the cyanation reaction occurred with complete syn diastereoselectivity using the matched pair. The versatility of the resulting amino nitrile as a synthetic intermediate was tested by performing the synthesis of orthogonally protected (R)-(2-aminomethyl)alanine.

Asymmetric synthesis and antimicrobial activity of some new mono and bicyclic beta-lactams

Reaction of the amino acid D-phenylalanine ethyl ester (4) with cinnamaldehyde gave chiral Schiff base 5, which underwent an asymmetric Staudinger [2+2] cycloaddition reaction with phthalimidoacetyl chloride to give the monocyclic beta-lactam 6 as a single stereoisomer. Ozonolysis of 6 followed by reduction with lithium aluminum tri(tert-butoxy) hydride afforded the hydroxymethyl beta-lactam 8. Treatment of 8 with methansulfonyl chloride gave the mesylated monocyclic beta-lactam 9, which was converted to the bicyclic beta-lactam 10 upon treatment with 1,8-diazabicyclo[5,4.0] undec-7-ene (DBU). Deprotection of the phthalimido group in beta-lactams 6 and 10 by methylhydrazine and subsequent acylation of the free amino beta-lactams with different acyl chlorides in the presence of pyridine afforded mono and bicyclic beta-lactams 14a-d and 15a-d respectively. The compounds prepared were tested against Escherichia coli, Staphilococcus citrus, Klebsiella pneumanie and Bacillus subtillis. Some of these compounds showed potential antimicrobial activities.

Rearrangement of 4-(1-haloalkyl)- and 4-(2-haloalkyl)-2-azetidinones into methyl omega-alkylaminopentenoates via transient aziridines and azetidines

The synthesis of 4-(1-haloalkyl)-2-azetidinones and 4-(2-haloalkyl)-2-azetidinones was investigated with use of the Staudinger reaction between in situ generated ketenes and alpha-haloimines or beta-haloimines. This new class of functionalized 2-azetidinones was further evaluated for its potential use as intermediates in the synthesis of highly functionalized compounds. The reaction of 4-(1-haloalkyl)-2-azetidinones and 4-(2-haloalkyl)-2-azetidinones with sodium methoxide in methanol yielded ring-opened products, i.e., methyl 2-alkoxy-4-(alkylamino)pentenoate and methyl 5-(alkylamino)pentenoate, respectively. Further attention was paid in detail to the reaction mechanism involved in this peculiar transformation. It was proven that these reactions proceeded via intermediate aziridines or azetidines.

Synthesis of Diazepinones via Intramolecular Transamidation

[structure: see text] The synthesis of a collection of bicyclic fused azepinones via an intramolecular beta-lactam ring-opening strategy is reported. Depending on the chirality of the various inputs, complete stereocontrol of product formation is achieved.

Synthesis of C-3' methyl taxotere (docetaxel)

Protected (3R,4S)-N-Boc-3-hydroxy-4-methyl-4-phenylazetidin-2-one has been synthesized stereoselectively and used to esterify protected 10-desacetylbaccatin III to give, following removal of the protecting groups, novel C-3' methyl taxotere (docetaxel).

Stereoselective synthesis of vancosamine and saccharosamine glycals via tungsten-catalyzed alkynol cycloisomerization

[reaction: see text] A stereoselective synthesis of the C-3 branched amino glycals of vancosamine and saccharosamine is described that features a tungsten carbonyl catalyzed cycloisomerization of the corresponding alkynyl alcohol.

Entry to New Conformationally Constrained Amino Acids. First Synthesis of 3-Unsubstituted 4-Alkyl-4-carboxy-2-azetidinone Derivatives via an Intramolecular Nα-Cα-Cyclization Strategy

A systematic study on the base-assisted intramolecular alkylation of N-benzyl-N-chloroacetyl amino acid derivatives is described. This study resulted in the first concise and versatile route to the preparation of 3-unsubstituted 4-alkyl-4-carboxy-2-azetidinones, to be included into the scarce family of beta-lactams with quaternary centers at the C(4) position. Particularly noteworthy is that the intramolecular N(alpha)-C(alpha)-cyclization of Phe and Leu derivatives afforded the corresponding beta-lactam derivatives with moderate enantioselectivity (up to 56%). It is suggested that, in these particular cases, the cyclization reaction proceeds by way of planar enolate intermediates, which possess dynamic chirality. The described sequence of reactions, that is compatible with commonly used protecting moieties for the alpha-carboxy group, cannot be applied to dipeptides, since the cyclization to the six-membered 2,5-diketopiperazine ring occurs preferentially.

Bis(trimethylsilyl)methylamine (BSMA): tool or toy?

Bis(trimethylsilyl)methylamine, BSMA, an original primary amine, was proven to be the source of a large variety of N-bis(trimethylsilyl)methylamino derivatives that otherwise have to be prepared indirectly. Because of the bulkiness of the bissilylmethyl group, regio- and stereoselectivities resulted in appropriated examples. Apart from the potential biological properties associated with the Si-C-N framework, these derivatives were powerful synthetic intermediates. Special emphasis was given to the chemistry of corresponding imines. Cleavage of Si—C bonds or abstraction of the methine proton afforded α-nitrogen carbanions in very mild conditions. Application of these results to the β-lactam chemistry allowed the introduction of new synthetic methodologies in this field. It results the (Me3Si)2CH- group could now be regarded as a Me3SiCH2- or a CH3- group or a proton equivalent.Key words: bis(silyl)methylamine, preparations, reactivity, nitrogen heterocycles, α-nitrogen carbanions.

Synthesis of novel 3'-C-methyl-apionucleosides: an asymmetric construction of a quaternary carbon by Claisen rearrangement

The synthesis of 2,3-dideoxy-3-C-(hydroxymethyl)-3-C-methyl-D-glycero-tetrofuranosyl++ + nucleosides was accomplished in high enatiomeric purity (98.5% ee) via [3,3]-sigmatropic Claisen rearrangement of (E)(S)-5-benzyloxy-1-tert-butyldimethylsilanyloxy-4-methyl-pent-3- en-2-ol prepared from 2,3-O-isopropylidene-D-glyceraldehyde. The synthesized nucleosides were assayed against human immunodeficiency virus (HIV) and hepatitis B virus in human peripheral blood mononuclear (PBM) and 2.2.15 cells, respectively. 6-Amino-9-[2,3-dideoxy-3-C-(hydroxymethyl)-3-C-methyl-beta-D-glycero- tetrofuranosyl]-2-fluoropurine shows moderate antiviral activity (EC50 = 2.55 microM) against HIV-1 strains and 6-amino-9-[3-deoxy-3-C-(hydroxymethyl)-3-methyl-alpha-D-glycero-tetro furanosyl]-2-fluoropurine exhibits potent anti-HIV activity (EC50 = 0.073 microM) with significant cytotoxicity (IC50 = 1.0 microM).

From beta-lactams to alpha- and beta-amino acid derived peptides

The potential of beta-lactams as intermediates for the access to alpha- and beta-amino acid-derived peptides is shortly reviewed, with major focus on the technologies developed in our group. The two general strategies lie, on one side, in the oxidative ring expansion of 3-hydroxy beta-lactams to N-carboxy alpha-amino acid anhydrides or Leuch's anhydrides and subsequent coupling with alpha-amino acid esters and, on the other side, in the nucleophilic ring opening of N-Boc-beta-lactams. Both approaches have been successfully applied to the synthesis of alpha,beta-diamino acid, alpha-amino-beta-hydroxy acid, polyhydroxylated alpha-amino acid, alpha,alpha-disubstituted alpha-amino acid, beta-amino acid, beta-amino-alpha-hydroxy acid and beta,beta-disubstituted beta-amino acid derived peptides. Because of the mild reaction conditions needed for the above transformations and the highly stereoselective procedures employed for the construction of the starting beta-lactam ring, the whole process allows the production of optically pure final products.

(3R)-Chiral Control of 3-Alkyl-3-hydroxy-beta-lactams via Addition Reaction of Imines to Enolates of 1,3-Dioxolan-4-ones

A method is described for the chiral construction of (3R)-3-alkyl-3-hydroxy-beta-lactams in a versatile and predictable manner. This protocol follows Seebach's synthetic principle of self-regeneration of stereocenters and has been applied to addition reactions among a selected number of imines and (2S)-chiral enolates of 1,3-dioxolan-4-ones. These reagents are easily available from the acetalization of (S)-alpha-hydroxy acids (lactic, mandelic, isovaleric, malic) and pivalaldehyde or pinacolone. In several cases, the addition of the enolate to the imine, the cyclization, and the removal of the auxiliary center occur in a one-step sequence, affording the corresponding beta-lactams as (3R,4S)-Z and (3R,4R)-E diastereomeric mixtures with high enantiomeric excesses. Four N-unsubstituted (3R,4S)-3-hydroxy-3-methyl-beta-lactams bearing 2-furyl (4e), phenylethenyl (4h), methoxycarbonyl (4i), and 2-thienyl (4l) substituents at C4 were obtained as major diastereomers and were purified by crystallization. The simultaneous presence of these substituents at C3 and C4 make these beta-lactams useful intermediates for the synthesis of new taxoids with interesting structural modifications at the isoserine moiety.