Synthesis Golf II: Erythronolide B A full FAQ post has been written on meta.chem.SE, explaining the premise of synthesis golf and the 'rules'. Please take a look at this if you haven't already. 



This round of synthesis golf concerns the Corey synthesis of Erythronolide B (J. Am. Chem. Soc. 1978, 100 (14), 4620–4622).



In order to narrow the scope of the challenge, only a small fragment of the natural product will be considered. Fragment 2 was an intermediate on the Corey route, and provided C10-C13 of Erythronolide B. 

Crucially, 2 was needed as a single enantiomer and diastereomer. In the original Corey report, 13 steps were needed to accomplish the synthesis of the fragment, a length which prevented significant amounts of material from being made. 

The challenge is to propose a synthesis of 2 which sets up the required 1,2-stereochemistry with absolute configuration. You may start from anything commercially available in the Sigma–Aldrich catalogue, including chiral building blocks.
I have established the stereochemistry via Evans aldol addition:

Then, after protection of the hydroxyl group, the chiral auxiliary can be transformed to a Weinreb amide which will be attacked by Grignard reagent:

The methyl ketone is then transformed to an alkyne which is methylated:
Finally, hydroboration (should be very regioselective) of the alkyne will provide the trans alkenylborane which on treatment with NaOH and iodine will provide target 12:


Length (total steps and longest linear sequence): 7 steps