This proposal seeks to exploit diastereoselective routes to bicyclic-[unreadable]-lactones to access antitumor, antibacterial and anti-inflammatory agents of interest for human health and as tools for studying basic biological processes. In this grant period, we will continue to exploit the potential of various methods developed in our group that enable concise and versatile total syntheses of several bioactive natural products including salinosporamides, scabrolides/ineleganolide, oxazolomycins, and haterumalides/biselides. These compounds all exhibit potent effects on various cell lines thus this project will also ultimately target the identification of their cellular targets where this is unknown or alternatively develop variants of these protein-reactive natural products as activity-based cellular probes to identify off-targets. The particular aims are: (1) Building on work from the previous grant period, optimization studies directed toward improved yields and diastereoselectivity of a concise (9 steps from serine) enantioselective synthesis of the potent proteasome inhibitor, salinosporamide A are proposed. Hypothesis-directed derivatives that may have increased potency are proposed based on the reported X-ray structures of the salino A- and belactosin-20S proteasome complexes (to be tested at Genzyme). (2) Building on our work toward the salinosporamides, we propose a synthesis of the oxazolomycin and neooxazolomcyin [unreadable]- lactam core involving modified bis-cyclizations. (3) We will exploit our recently developed, diastereoselective bis-cyclization reaction of keto acids to access the cyclopentyl core common to the scabrolide/ineleganolide family of marine cembranes. To construct, the bicyclic-[unreadable]-lactone macrocyclic core, we propose a transannular CH insertion that would be of fundamental interest for the synthesis of this growing family of bicyclic "-lactone macrocycles and related targets. (4) We propose double- diastereoselective, bis-cyclizations for the synthesis of [unreadable]-lactone-fused tetrahydrofurans to access THFs found in the haterumalides(Hat)/biselides(Bise).