BU3SNH-MEDIATED PINACOL COUPLING OF 1,5-DICARBONYL AND 1,6-DICARBONYLCOMPOUNDS - SYNTHETIC AND MECHANISTIC STUDIES

A new method is described for the intramolecular pinacol coupling of 1 ,5- and 1,6-dicarbonyl compounds, employing Bu3SnH as the stoichiometr ic reductant. The key steps in this pinacol cyclization are the additi on of a tin ketyl to a carbonyl group and a subsequent intramolecular S(H)2 reaction. The isolation of 1,3-dioxa-2-stannolanes, along with o ther product and labeling studies, provides strong support for the pro posed homolytic substitution step, which distinguishes the pinacol cyc lization from other reductive cyclizations of tin ketyls, all of which proceed through abstraction of hydrogen from Bu3SnH in the final step . An interesting consequence of the S(H)2 pathway is very high cis sel ectivity in the cyclization of 1,5-dicarbonyl compounds. Mechanistic s tudies furnish evidence that the steps that precede homolytic substitu tion, including C-C bond formation, are reversible under the reaction conditions.