Synthesis of the C(3)C(15) segment of rutamycin B via a C(8)C(9) fragment assembly aldol reaction: Metal dependence of the aldehyde and enolate diastereofacial selectivities

A concise total synthesis of saliniketal B

We report a concise, enantioselective, and highly efficient synthesis of the marine actinomycete-derived natural product saliniketal B. Our approach was motivated with an eye toward future structure-function studies of this inhibitor of phorbol ester-mediated ornithine decarboxylase induction via an unknown mechanism. Our strategy highlights the utility of Pt(II)-mediated cycloisomerization of alkynediols developed in our laboratory to construct the dioxabicyclo[3.2.1]octane ring system, a highly selective aldol fragment coupling whose stereochemical outcome is influenced by a gamma-stereogenic methyl group, and an interesting one-pot desilylation/dihydropyranone fragmentation/amidation sequence. As such, saliniketal B was obtained in 11 steps and 23% overall yield from commercially available starting material via a convergent coupling of two equally complex fragments assembled in seven and eight steps (39 and 45%), respectively.

Diastereodivergent aldol reactions of beta-alkoxy ethyl ketones: modular access to (1,4)-syn and -anti polypropionates

Asymmetric substrate-controlled aldol reactions of ethyl ketones of type 4 with aldehyde 3 are reported. Modular access to all possible syn- and anti-aldol products was obtained by careful choice of reaction conditions. To achieve good selectivities in this diastereodivergent approach, selection of the protective group on the beta-oxygen of the enolate (R (2)) was of critical importance.

Studies on the synthesis of bafilomycin A(1): stereochemical aspects of the fragment assembly aldol reaction for construction of the C(13)-C25) segment

Highly stereoselective syntheses of aldols 8a-c corresponding to the C(13)-C(25) segment of bafilomycin A(1) were developed by routes involving fragment assembly aldol reactions of chiral aldehyde 6a and the chiral methyl ketones 7. A remote chelation effect plays a critical role in determining the stereoselectivity of the key aldol coupling of 6a and the lithium enolate of 7b. The protecting group for C(23)-OH of the chiral aldehyde fragment also influences the selectivity of the lithium enolate aldol reaction. In contrast, the aldol reaction of 6a and the chlorotitanium enolates of 7a,c were much less sensitive to the nature of the C(15)-hydroxyl protecting group. Studies of the reactions of chiral aldehydes with Takai's (gamma-methoxyallyl)chromium reagent 40 are also described. The stereoselectivity of these reactions is also highly dependent on the protecting groups and stereochemistry of the chiral aldehyde substrates.

Total synthesis of rutamycin B, a macrolide antibiotic from Streptomyces aureofaciens

Rutamycin B (2) was synthesized from three principal subunits, spiroketal 75, keto aldehyde 83, and aldehyde 108. First, triol 62 was assembled by Julia coupling of sulfone 56 with aldehyde 58 followed by an acid-catalyzed spiroketalization. The three hydroxyl functions of 62 were successfully differentiated, leading to phosphonate 75. The latter was condensed in a Wadsworth-Emmons reaction with 83, prepared in six steps from (R)-aldehyde 76, to give 92. Coupling of the titanium enolate of 92 with 108 afforded Felkin product 109 with high stereoselectivity in a process that is critically dependent on the presence of the p-methoxybenzyl ether in the aldehyde. Transformation of 109 via aldehyde 116 to vinylboronate 122 was followed by macrocyclization under Suzuki conditions to yield 123. Exhaustive desilylation of the latter yielded rutamycin B.

Studies on the total synthesis of formamicin: synthesis of the C(1)-C(11) fragment

[figure: see text] An efficient and highly concise synthesis of 6, corresponding to the C(1)-C(11) fragment of formamicin (1), has been accomplished by a route utilizing a diastereoselective lactate aldol reaction to set the C(6) tertiary ether and the TES-OTf mediated transketalization of the C(6) tertiary methoxymethyl ether and the C(25) PMB ether to set the seven-membered methylene acetal unit (see 37-->38).