Enantio- and diastereoselective additions to aldehydes using the bifunctional reagent 2-(chloromethyl)-3-(tributylstannyl)propene: application to a synthesis of the C16-C27 segment of bryostatin 1

Highly Diastereoselective Chelation-Controlled 1,3-anti-Allylation of (S)-3-(Methoxymethyl)hexanal Enabled by Hydrate of Scandium Triflate

En route to the total synthesis of (+)-Neopeltolide, we explored Lewis acid-assisted diastereoselective allylation of MOM-protected 3-hydroxylhexanal with β-(2,2-diethoxyethyl)-substituted (allyl)tributylstannane. The hydrated form of scandium triflate was found to be essential for attaining high 1,3-anti-diastereoselectivity (d.r. 94:6), while the use of anhydrous catalyst resulted in a modest diastereocontrol (d.r. 76:24). The preferred 1,3-anti-selectivity in this transformation can be rationalized in the framework of the Reetz chelate model of asymmetric induction. The 1,3-anti-configuration of the product was confirmed by its conversion into the known C7-C16 building block of (+)-Neopeltolide. We also report an improved protocol for the synthesis of β-(2,2-diethoxyethyl)-substituted (allyl)tributylstannane, which can be utilized as a cost-efficient bipolar isoprenoid-type C5-building block in the synthesis of natural compounds.

Au(I)-Catalyzed Cyclization of Epoxyalkynes to Allylic Alcohol Containing Spiroketals and Application to the Total Synthesis of (-)-Alotaketal A

A gold-catalyzed tandem spiroketalization of epoxyalkynes accompanied by epoxide rearrangement into the corresponding allylic alcohol was developed for the construction of functionalized spiroketals. This new synthetic methodology for unsaturated spiroketals warranted a facile total synthesis of alotaketal A from carvone via a corresponding epoxyalkyne precursor.

Successive Nucleophilic and Electrophilic Allylation for the Catalytic Enantioselective Synthesis of 2,4-Disubstituted Pyrrolidines

Successive nucleophilic and electrophilic allylation mediated by the bis-Boc-carbonate derived from 2-methylene-1,3-propane diol enables formation of enantiomerically enriched 2,4-disubstituted pyrrolidines. An initial enantioselective iridium-catalyzed transfer hydrogenative carbonyl C-allylation is followed by Tsuji-Trost N-allylation using 2-nitrobenzenesulfonamide. Subsequent Mitsunobu cyclization provides the N-protected 2,4-disubstituted pyrrolidines.

Enantioselective Ir-Catalyzed Bidirectional Reductive Coupling

In the presence of a chiral iridium complex, commercially available 3-chloro-2-chloromethyl-1-propene (1) was selectively activated for various reductive couplings. Depending on the reaction conditions it allows a selective mono- or bidirectional condensation with one or two external aldehydes with excellent enantiocontrol (>90% ee). This approach occurring simply under mild conditions and avoiding premetalated reagents constructs rapidly chiral homoallylic alcohols, key precursors of important molecular fragments such as furans, pyrans, ketodiols, or 1,3,5-polyols.

Synthesis and in vitro evaluation of analogues of avocado-produced toxin (+)-(R)-persin in human breast cancer cells

A structure-activity study of several new synthetic analogues of the avocado-produced toxin persin has been conducted, with compounds being evaluated for their cytostatic and pro-apoptotic effects in human breast cancer cells. A 4-pyridinyl derivative demonstrated activity comparable to that of the natural product, suggesting future directions for exploration of structure-activity relationships.

Total synthesis of bryostatins: the development of methodology for the atom-economic and stereoselective synthesis of the ring C subunit

Bryostatins, a family of structurally complicated macrolides, exhibit an exceptional range of biological activities. The limited availability and structural complexity of these molecules makes development of an efficient total synthesis particularly important. This article describes our initial efforts towards the total synthesis of bryostatins, in which chemoselective and atom-economical methods for the stereoselective assembly of the ring C subunit were developed. A Pd-catalyzed tandem alkyne-alkyne coupling/6-endo-dig cyclization sequence was explored and successfully pursued in the synthesis of a dihydropyran ring system. Elaboration of this methodology ultimately led to a concise synthesis of the ring C subunit of bryostatins.

Total synthesis of epothilones B and D: stannane equivalents for beta-keto ester dianions

Studies leading to a total synthesis of epothilones B and D are described. The overall synthetic plan was based on late-stage fragment assembly of two segments representing C(1)-C(9) and C(10)-C(21) of the structure. The C(1)-C(9) fragment was prepared by elaboration of commercially available (2R)-3-hydroxy-2-methylpropanoate at both ends of the three-carbon unit. Introduction of carbons 1-4 containing the gem-dimethyl unit was achieved in a convergent manner using a diastereoselective addition of a stannane equivalent of a beta-keto ester dianion. An enantioselective addition of such a stannane equivalent for a beta-keto ester dianion was also used to fashion one version of the C(10)-C(21) subunit; however, the fragment assembly (using bimolecular esterification followed by ring-closing metathesis) with this subunit failed. Therefore, fragment assembly was achieved using a Wittig reaction; this was followed by macrolactonization to close the macrocycle. The C(10)-C(21) subunit needed for this approach was prepared in an efficient manner using the Corey-Kim reaction as a key element. Other key reactions in the synthesis include a stereoselective SmI(2) reduction of a beta-hydroxy ketone and a critical opening of a valerolactone with aniline which required extensive investigation.

Synthesis of substituted benzenes and phenols by ring-closing olefin metathesis

New synthetic approaches to substituted aromatic compounds are reported. Ring-closing olefin metathesis (RCM)/dehydration and RCM/tautomerization are the key processes in the synthesis of substituted benzenes 3 and phenols 6, respectively. Readily accessible 1,5,7-trien-4-ols 7, which are the precursors of benzenes, were prepared from beta-halo-alpha,beta-unsaturated aldehydes 11 or beta-halo-alpha,beta-unsaturated esters 19 by utilizing reliable transformations in which cross-coupling with vinylic metal reagents 12 and allylation with allylic metal reagents 13 were employed as carbon-carbon bond forming reactions. RCM of 7, followed by dehydration, afforded a wide variety of substituted benzenes 3. In addition, RCM of 1,5,7-trien-4-ones 9, which were prepared by oxidation of 7, furnished various substituted phenols 6 by automatic tautomerization.

A catalytic asymmetric vinylogous Mukaiyama aldol reaction

A vinylogous Mukaiyama aldol reaction, conducted using 10 mol % of a BITIP catalyst and B(OMe)3 as an additive, effects an enantioselective four-carbon chain extension to give versatile E-alpha,beta-unsaturated thiol esters.

Synthetic studies toward the bryostatins: a substrate-controlled approach to the A-ring

[reaction: see text] The synthesis of a C1-C13 A-ring subunit of bryostatin 1 is detailed. The key features of the approach include the convergent fragment assembly with a highly stereoselective construction of the C7-C8 bond indicated above.

A new construction of 2-alkoxypyrans by an acylation-reductive cyclization sequence

A new convergent synthetic approach to a pyran motif common to many naturally occurring structures is described. In this approach, two fragments are joined by esterification, and a subsequent intramolecular reductive cyclization affords the 2-hydroxypyran.

Synthetic Studies Toward Bryostatin 1: Preparation of a C(1)-C(16) Fragment by Pyran Annulation

An expeditious assembly of a C(1)-C(16) subunit of bryostatin 1 is described. A pyran annulation reaction was utilized to form the B-ring by reaction of a hydroxy-allylsilane with a fully elaborated A-ring subunit. This annulation process proceeded with complete diastereoselectivity and in excellent isolated yield despite the presence of potentially sensitive functionality in the A-ring segment.

Synthetic studies on the bryostatins: preparation of a truncated BC-ring intermediate by pyran annulation

[reaction: see text]. A synthesis of a potential BC-ring subunit (C9-C27) for bryostatin 1, a remarkably potent anticancer agent, has been developed in 16 steps and 18% overall yield. The key features of this route include a BITIP-catalyzed asymmetric allylation reaction, chelation-controlled allylations, a hydroformylation reaction, and a pyran annulation reaction.