Cyclization-pinacol sequence

On the basis of this rearrangement, an efficient synthetic route to pyrrolidines such as (46) was developed by Overman, starting from oxazolidine (42), via hydroxylated homoallylic imines such as (43 Scheme 22).Mechanistically, the formation of (46) may explained as a tandem-type combination of a cationic aza-Cope rearrangement with a subsequent Maimich cyclization (route a) or, alternatively, as an alkene-iminium ion cyclization/pinacolic rearrangement sequence (route b). 

The cascade synthesis of tetrahydrofuran-containing oxacyclic molecules takes place by a 2-oxonia[3,3]-sigmatropic-aldol mechanism rather than by a Prins cyclization-pinacol rearrangement sequence (Scheme 6). ... 

Ring expansions of appropriately a-substituted ketones via their vinyl carbinol derivatives has also been carried out under cationic conditions. The basic sequence, outlined in Scheme 66, begins with the addition of a vinyl organometallic to an a-substituted ketone, where X is carbon or a heteroatom. Departure of a leaving group Y then produces a cationic species, which may react further to form products. Two main pathways appear to be operative one is a cationic alkene cyclization, followed by a pinacol-like rearrangement while the other is a 3,3 or 3,3-like rearrangement, followed by an intramolecular alkylation of the intermediate enol. 

Compound (80), prepared from 2-methylcyclopentane-l,3-dione and methyl 2-chloroacrylate followed by the sequence (76)-> (77)-> (78)- (79) (resolved)— (80), combined with (71) (prepared from m-methoxyphenylstyrene and diborane) to form the seco-steroid (82)/ Acetic anhydride-toluene-p-sulphonic acid then cyclized this stereoselectively to furnish the triacetate (83) which on saponification gave the triol (84). Interestingly, this compound reacted with toluene-p-sulphonic acid in alcohol to produce, by dehydration and change of configuration at C-14, the compound (85), which served as a source of various 8a-oestrone compounds. Compound (84) on treatment with boron trifluoride etherate underwent pinacol transformation in preference to dehydration to yield the ketone (86) this ketone was correlated with the known compound (87). 

In this lecture I will summarize recent investigations at Irvine concerning the use of pinacol rearrangements to terminate cationic cyclizations.[5] The overall sequence is illustrate in Figure 1, and has been employed to prepare... 

A microwave procedure for tbe synthesis of a focused library of 3-amino-imid-azopyridines bas been developed by DiMauro and Kennedy (2007). Imidazopyri-dine products were obtained via an Ugi-type cyclization, starting from 2-aminopyri-dine-5-boronic acid pinacol ester, where an intermediate was formed. It was further converted into the final product by a Suzuki coupling in a one-pot procedure. It was interesting to note that it was neither possible to perform the reaction sequence in one step nor the sequence can be reversed. 

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