Stereocontrolled synthesis of α-trialkylsilyl-β, γ-unsaturated aldehydes via palladium (0) catalysis. Synthetic usefulness

Silylpyrrole Oxidation En Route to Saxitoxin Congeners Including 11-Saxitoxinethanoic Acid

Saxitoxin (STX) is the archetype of a large family (>50) of architecturally distinct, bisguanidinium natural products. Among this collection of isolates, two members, 11-saxitoxinethanoic acid (11-SEA) and zetekitoxin AB (ZTX), are unique, bearing carbon substitution at C11. A desire to efficiently access these compounds has motivated the development of new tactical approaches to a late-stage C11-ketone intermediate 26, designed to enable C-C bond formation using any one of a number of possible reaction technologies. Highlights of the synthesis of 26 include a metal-free, silylpyrrole oxidative dearomatization reaction and a vinylsilane epoxidation-rearrangement cascade to generate the requisite ketone. Nucleophilic addition to 26 makes possible the preparation of unnatural C11-substituted STXs. Olefination of this ketone is also demonstrated and, when followed by a redox-neutral isomerization reaction, affords 11-SEA.

A convenient multigram synthesis of highly enantioenriched methyl 3-silylglycidates

A multigram scale synthesis of the four stereoisomers of methyl 3-silylglycidates (epoxysilanes) with high enantiopurity is described. Key reactions include a Sharpless asymmetric epoxidation (SAE) of a trans-vinylsilane and an enzymatic resolution of a racemic cis-epoxysilane to establish the desired configurations. Few chromatographic separations (5 columns out of 13 steps) are required for purification, establishing a convenient reaction sequence for both the trans- and cis-isomers.

New efficient construction of the ABC core of the taxoids via a sequence of consecutive cobalt(I)-mediated [2 + 2 + 2] and [4 + 2] cyclizations

[reaction: see text] A new and efficient sequence of two consecutive cyclizations, a cobalt(I)-mediated [2 + 2 + 2] cyclotrimerization and a Diels-Alder reaction, is proposed to allow the formation of the ABC core of the taxoids in 65% overall yield, starting from an acyclic polyunsaturated precursor.

Enantioselective allylation of beta,gamma-unsaturated aldehydes generated via Lewis acid induced rearrangement of 2-vinyloxiranes

[reaction: see text] 2-Vinyloxiranes have been found to be excellent surrogates to beta,gamma-unsaturated aldehydes. These valuable electrophiles, generated in situ by treatment of a 2-vinyloxirane with a catalytic amount of Sc(OTf)(3), are effectively trapped by the chiral allylating agents based on alpha-pinene, affording bishomoallylic alcohols in high yield and excellent selectivity.