Bimetallic Reagents of Silicon: One‐Pot Synthesis of 2,3,5‐Trisubstituted Tetrahydrofurans by a Double Sakurai–Hosomi Reaction

Abstract

Unsaturated bimetallic reagents containing Group 14 metals, such as silicon and tin, have recently emerged as versatile coupling reagents in organic synthesis. [1, 2] These reagents are particularly appealing when both metals employ a "ferryman" service to yield metal-free cyclic products in a one-pot reaction. However, examples of such reactions are rare. [2] As part of our interest in allyl organometallic compounds of silicon, [3] we envisioned that 1-silylmethyl allylic silanes 1 could be useful synthetic building blocks. In the presence of a Lewis acid, a bis-silyl reagent such as 1 affords a b-silyl carbocation 2 (Scheme 1) when treated with an aldehyde. This carbocation may then collapse to give an oxetane 3 (path a) [4] or, more likely, undergo a 1,2-silyl migration to give the thermodynamically favored bis-b-silyl carbocation 4, [5] which produces a trisubstituted tetrahydrofuran 5 (path b).[6] The latter was realized recently by Peng and Woerpel. [7] Additionally, a third pathway is also conceivable wherein 2 collapses to give a new allylic silane 6. The allylic silane 6 should be less reactive than the starting allylic silane 1 because its p bond is more hindered. Introduction of a second aldehyde should yield a silicon-free tetrahydrofuran 8 by trapping of the oxocarbenium ion 7 (path c). While our studies were in progress Smitrovich and Woerpel [8] reported a single experiment on the synthesis of a tetrahydrofuran lacking silyl groups by reaction of a substituted silylmethyl allylic silane and an aldehyde; however, a pure product could not be obtained and a furan structure was only tentatively assigned. Herein, we provide the first report that silylmethyl allylic silanes of type 1 can be channeled through path c to yield only 2,3,5-trisubstituted tetrahydrofurans.As the 1-silylmethyl allylic silanes 1 a,b (Scheme 2) selected for our study are novel, there were no reported procedures for their synthesis. Smitrovich and Woerpel experienced great difficulties in preparing similar compounds but eventually succeeded by employing allylic displacement, [9] which unfortunately could not be applied in our case since only carbamates from secondary allyl alcohols are amenable to it. Thus, a convenient synthetic methodology to this family of allylic silanes was our initial task. Our successful route (Scheme 2) involved heating disilanyl ether 9 (prepared in

A Highly Stereoselective Synthesis of Tetrahydrofurans

“…However, the starting homoallyl alcohol has to be prepared prior to the actual cyclization in a separate step. A significant advance toward a more direct domino process came with the development of bis(allylsilane)s from Sarkar and Santelli, which merged the preparation of the homoallyl alcohol with the Prins cyclization. However, these procedures were both limited to the use of identical aldehydes and hence equal 2‐ and 5‐substituents within the tetrahydrofuran and provided merely racemic products.…”

Recent advances in the stereoselective synthesis of tetrahydrofurans

“…For example, tetrahydrofuran 77 was prepared from aldehyde 75 and allylsilane 74 in 68% yield as a single diastereomer via intermediate 76 (eq 26). 46 Yamamoto has described the formation of 2-vinyl-3-hydroxytetrahydrofurans from γ-alkoxyallylstannanes bearing tethered aldehydes. 47 Either product stereoisomer can be obtained with good selectivity using the appropriate reaction conditions.…”

Reaction of benzoxasilocines with aromatic aldehydes: Synthesis of homopterocarpans

“…[2,3] We have applied it to the stereoselective synthesis of dihydrobenzofurans by means of the condensation of benzoxasilepines with aromatic aldehydes in the presence of Lewis acids. [4,5] Using this methodology and through convergent synthetic routes, we have prepared pterocarpans [6] and neolignans.…”