Unusual Rearrangement-Remercuration Reactions of Allylic Silanols

Dancing Silanols: Stereospecific Rearrangements of Silanol Epoxides into Silanoxy-Tetrahydrofurans and Silanoxy-Tetrahydropyrans

We have developed highly stereospecific rearrangements of silanol epoxides into 1'-silanoxy-tetrahydrofurans and 1'-silanoxy-tetrahydropyrans. Upon treatment with Ph3CBF4 and NaHCO3 in CH2Cl2, di-substituted trans-epoxide silanols rearrange into products with an erythro configuration; di-substituted cis-epoxide silanols give products with a threo configuration. We have used these reactions as key steps in the syntheses of (±)-solerone and (±)-muricatacin.

Covalent Tethers for Precise Amino Alcohol Syntheses: Ring Opening of Epoxides by Pendant Sulfamates and Sulfamides

We describe the development of the first ring opening of epoxides using pendant sulfamates and sulfamides. These reactions are promoted by a base and proceed under mild conditions to afford oxathiazinanes and cyclic sulfamides with excellent diastereoselectivity and regiocontrol. The reactions scale well, and the products serve as synthons for ring-opening reactions.

Ring Opening of Aziridines by Pendant Silanols Allows for Preparations of (±)-Clavaminol H, (±)-Des-Acetyl-Clavaminol H, (±)-Dihydrosphingosine, and (±)-N-Hexanoyldihydrosphingosine

We present a unique strategy for the synthesis of vicinal amino alcohols. Ring opening of aziridines with pendant silanols is compatible with a range of substrates. To engage productively in ring opening, the aziridine must be at least mildly activated, and a variety of such N-substituents are tolerated. The utility of this methodology is highlighted in facile preparations of the natural products (±)-Clavaminol H, (±)-dihydrosphingosine, and (±)-N-hexanoyldihydrosphingosine as well as a natural product analogue (±)-des-acetyl-Clavaminol H.

Highly Stereospecific Cyclizations of Homoallylic Silanols

We demonstrate that di-tert-butylsilanols are competent nucleophiles for the intramolecular interception of palladium π-allyl species. In these reactions, allyl ethyl carbonates are the best precursors for the formation of palladium π-allyl intermediates, and [(Cinnamyl)PdCl]₂/BINAP is superior to other Pd salt/ligand framework combinations. Our optimized protocol is compatible with a variety of silanol substrates. Importantly, the cyclization is perfectly stereospecific, proceeding via an anti-syn mechanism, which stands in contrast to reported analogous reactions of alcohols and phenols, known to proceed via an anti-anti mechanism. The alkenes in the product dioxasilinanes serve as blank slates for further functionalization.

Hydroxyselenylation and Tethered Silanoxyselenylation of Allylic Silanols

We present protocols for the highly regioselective hydroxyselenylation and silanoxyselenylation of allylic silanols. N-(Phenylseleno)phthalimide acts as the selenylating agent for both transformations. Under basic conditions, hydroxyselenylation proceeds with >20:1 regioselectivity, and the products are valuable synthons for further transformations. We show that the silanol plays a critical role in maintaining the yield and regioselectivity of this reaction. Surprisingly, under acidic conditions, the hydroxyselenylation pathway is blocked, and products of a tethered silanoxyselenylation are exclusive.

Ring Opening of Epoxides by Pendant Silanols

We present a new ring-opening reaction of epoxides by pendant silanols, catalyzed by either Ph₃C⁺BF₄^- or BINOL-phosphoric acid. Silanol epoxides derived from trans-allylic alcohols, cis-allylic alcohols, trans-homoallylic alcohols, and cis-homoallylic alcohols were all compatible and gave products from either endo- or exo-ring opening. With silanol epoxides derived from 4-alkenyl silanols, an unusual rearrangement to tetrahydrofuran products was observed. The utility of this methodology was demonstrated in a short preparation of protected d-arabitol.