Cyclic vinylsilanes, synthesis

Mallya, M. N., Nagendrappa, G. cis-Hydroxylation of cyclic vinylsilanes using cetyltrimethylammonium permanganate. Synthesis 1999, 37-39. 

Aryl- and allylsilanes reacted with alkynes intramolecularly in the presence of HfCU catalyst to afford the corresponding trans-carbosilylated cyclic compounds. The reaction of phenylsilylalkyne (149) proceeded in exclusively exo-cyclization mode to afford cyclic ( )-vinylsilane (150) in a good yield (Equation 65) [71]. In the case of silylated enyne (151), only endo-dig cyclization proceeded to afford vinylcyclohexene (152) in excellent yield (Equation 66) [72]. The reaction was applied for the synthesis of disilanylene-containing polymers [73]. 

Acylsilanes are yet another class of valuable synthetic intermediates, and new methods for their synthesis are always welcome. In the first of several examples reported this year, the silyl allene (168) has been used as a common intermediate for the preparation of a diverse range of acylsilanes with a,p-olefinic, a,p-acetylenic, and a-keto substituents. Alkyl-substituted acylsilanes are now available via a high-yielding process in which an acyl-lithium (generated in situ from an alkyl-lithium and CO) is trapped by MeaSiCl. Cyclic acylsilanes have been prepared from cyclic vinylsilanes by way of an epoxidation-reductive ringopening-oxidation sequence. ... 

The synthesis of L-daunosamine began with the condensation of trans-crotonaldehyde (56) with dibenzylhydrazine (Scheme 17). Sharpless asymmetric dihydroxylation of the resulting ( )-a, (3-unsaturated hydrazone 57 afforded the syn-diol 58 (70% yield, 89% ee by HPLC), and silylation with chlorodimethyl-vinylsilane then provided the radical cyclization precursor 59 in 98% yield. In the key step, exposure to thiyl radicals generated from PhSH and AIBN led to radical cyclization of dibenzylhydrazone 59. The unstable cyclic intermediate was then directly treated with fluoride to afford vinyl adduct 60 in 77% yield (dr 91 9, H NMR). In control experiments with corresponding monosilyl derivatives, the (3-O-silyl... 

Through the efforts of the Overman research group, the acetal-vinylsilane cyclization reaction has been shown to be a useful strategy for the stereoselective preparation of a variety of medium-sized oxygen heterocycles. This cyclization strategy has been used successfully in the asymmetric synthesis of the marine natural product (-)-laurenyne (40a), an eight-membered cyclic ether. These molecules are members of an unusual class of C-15 nonisoprenoid metabolites. 

Vinylsilanes as nucleophilic terminators offer several additional advantages over ordinary nonactivated alkenes (c/. Section 4.2.2.1). The silyl group is readily substituted stereo- and regio-selectively by the electrophilic carbon atom of the oxocarbenium ion, as demonstrated in Overman s synthesis of alkyl-idenetetrahydropyran (109 Scheme 53). This strategy was also applied to the preparation of five- and seven-membered cyclic ethers. ... 

Eight- and nine-membered cyclic ethers are also generated via type-III reactions by treating the 5-hexen-l-ol acetals with Lewis acids. 8-endo Cyclization of 5-hexenyl methoxyethoxymethyl ether in the presence of two equivalents of tin(IV) chloride forms a 2 1 mixture of chlorinated and unsaturated 1 -oxycyclooctanes 11 and 1232. Vinylsilane undergoes an 8-endo Prins cyclization. After O-desilylation, oxocene 13, which is used as a precursor in a laurenyne synthesis, is obtained in 37% yield33. 

Overman has also shown that eight- and nine-membered cyclic ethers can be prepared by type 111 cy-clizations.136 Treatment of (257) with 2 equiv. of SnCL for 13 h at -20 C gives (258), which cyclizes to give a 2 1 mixture of (259) and (260) in 83% yield.137 Cleaner reaction mixtures have been obtained using vinylsilanes, as in the synthesis of laurenyne.138 SnCU-mediated cyclization of acetal (261) in CH2CI2 at 0 C, followed by 0-desilylation produces oxocene (262) in 37% yield as the sole cyclic ether (Scheme 38). 

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