Carbocyclic structures stereoselectivity

The present [3 + 4] annulation methodology for the synthesis of seven-membered carbocycles involves a straightforward procedure that also provides the product functionalities (e.g., masked and unmasked ketone carbonyl and trimethylsilyl groups) that can be transformed to hitherto inaccessible or difficult-to-prepare cycloheptane structures. The prior approach,7-9 based on Cope rearrangement of cis-1,2-divinylcyclopropane, bears an intrinsic drawback in that there exist a limited number of methods for stereoselective preparation of the substrate. 

Once the structure was known, many syntheses were reported and all of a sudden the literature was loaded with stereoselective methods for olefin synthesis. Although it is only a guess, it is probable that the discovery of Cecropia juvenile hormone (CJH) was responsible (in part) for this burst of activity. Another reason would be the interest in carbocycle synthesis via polyolefin cyclizations (the development of steroid and terpene chemistry) which slightly preceeded the discovery of CJH, We will start by looking at the Trost group synthesis (the first synthesis) of CJH and then examine several other syntheses wherein the focus... 

Prins cyclizations, which proceed by intramolecular addition of alkenes to oxocarbenium ions, provide a simple, efficient method for the stereoselective synthesis of carbocycles and cyclic ethers [77]. Halosilanes and (la) have been used for Prins cyclizations not only as Lewis acids but also as heteroatom nucleophiles. For instance, in the presence of MesSil or MesSiBr, and lutidine, mixed acetals (26) are efficiently cyclized to 4-halotetrahydropyrans (27) with high diastereoselectivity [78]. The halide is introduced into the axial site of the C(4) position. The proposed mechanism for the MesSiBr-promoted reaction involves the initial formation of a-bromoethers (28) from (26). Solvolysis of (28) provides the intimate ion pair (29). Cyclization to the chair transition structure (30) and proximal addition of the bromide produces the observed axial adduct (27). The role of lutidine is to suppress a less selective HBr-promoted cyclization (Scheme 9.23). Acetals bearing an alkyne or allene moiety also undergo the halosilane-promoted cyclization to form haloalkenes [79, 80]. 

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