The remaining three tactical conundrums could be solved as summarized in Scheme 1B. With copious quantities of the key proline fragment in hand, its union with a suitably functionalized diamino synthon (16) was explored. Standard polar bond analysis suggested that a Mannich-type reaction of b-keto ester 18 with imine 19 followed by decarboxylation would be logical . Unfortunately, despite numerous attempts this transformation was unworkable.