Decalins polyene cyclization

The main disadvantage of the mercuronium ion induced polyene cyclizations is that mostly mixtures of mono-, bi-, tri-, etc. cyclic products are observed. Several examples generating trans-decalin structures via two consecutive 6-endo steps in moderate to useful yields are shown below101 ... 

In contrast to (Lewis) acid-catalysed annulations of /3-keto-esters such as (222), which lead to decalins cf. refs. 175,176), mercuric ion-induced cyclization of (222) leads instead to the pyran (223). Further examples of the ability of a carbon-tin bond to provide a specific nucleophilic site in TiCU-catalysed cyclizations of allylic alcohols have been reported, together with a full account of the use of an imino group to initiate polyene cyclizations annulations of imines that carry a chiral substituent result in moderate chiral inductions. 

This methodology has been applied to a highly stereoselective biomimetic polyene cyclization using chiral tricarbonyl(ii -l-pentadienol)iron complexes to give rise to franj-decaline (Ti -diene)iron complexes. The transformation represents a potential route to bicyclic sesquiterpenes and tricyclic diterpenes (Scheme 4-191). ... 

The feasibility of cyclizing polyenes with iminium ion initiators has received only scant attention. 72 Cyclization of imine (77) under aprotic conditions with SnCU affords predominantly rran -decalins containing endocyclic unsaturation. This mixture was deduced by H NMR analysis to contain diastereomers (78) to (81) in the indicated abundances as depicted in equation (7). The extent of asymmetric induction in forming the decalin ring system (9S,10iS 9/ ,10/ = 61 39) is significantly lower than that of related cyclizations of chiral acetal substrates. ... 

If the substituents are arranged to form a frans-decalin with complementary substitution, no cyclization occurs (Eq. 57) [17]. The slower six-membered ring formation at the hindered alkene is interrupted by silylation. This allows impressive chemoselectivity in polyene substrates. When presented with a substrate possessing three monosubstituted olefins with varying steric environments, the catalyst can select the least hindered alkene for initial insertion and will only cyclize onto the less hindered of the two remaining double bonds (Eq. 58). A mechanistic outline and depiction of the steric environments is given in Fig. 11. 

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