A Facile Access to Densely Functionalized Substituted Cyclopentanes and Spiro Cyclopentanes. Carbocation Stabilization Directed Bond Migration in Rearrangement of Cyclobutanes

Substituted cyclopentanes are widely represented in nature and have served as valuable precursors1 in the total synthesis of natural products. The cyclopentanone I having substituents at the 2, 3, and 5 positions as present in the novel sesquiterpene herbadysidolide II2 and related compounds poses considerable synthetic challenge. Although much effort has been spent on annulation of cyclopentane rings3 onto preexisting rings, little attention has been given to the synthesis of cyclopentanones4 with multiple substitution as represented by the structure I. Construction of cyclopentanones of this type through stepwise addition of substituents to prebuilt cyclopentanone is a lengthy process and is in general attended with poor regio- and stereoselectivity. An approach in which substituents are directly generated during construction of the cyclopentane ring is likely to have great synthetic importance in terms of selectivity and step economy. Development of a general methodology to construct cyclopentanones represented by the general structure I, thus, became our primary objective. We anticipate that acid-induced rearrangement5 of the cyclobutane ring in the tricyclic compound III (R3,R4 ) O) may produce the desired cyclopentanone I provided 1,5-bond migration takes place preferentially over the 1,7-bond. In light of the stereoelectronic requirement6 in the pinacol rearrangement, the 1,7-bond migration may be competitive. Thus it is of considerable interest to investigate the rearrangement of the cyclobutane derivatives III leading to a general route for the direct synthesis of cyclopentanones with functionalised substituent