General Consideration of the Petasis Reaction

When optically pure aldehyde was used, stereocontrolled back-side boronation of the intermediary complex X assured formation of a,p-aminoalcohol XI in the trans relative configuration and defined the absolute configuration at the second stereogenic centre. 

Reagents and conditions /. soivent, r.t. //. TMS-CHN2 racemate is presented) 

When this reaction was completed in hexafluoroisopropanol (HFIP), the diastereomeric ratio was enhanced to approximately 95 5 (90.0% d.e.), and the reaction time shortened from a few days to 20 h. To establish the relative 1,3-trans stereochemistry between the two stereogenic centres, a single crystal X-ray structure was obtained for hydrochloride 7 (as the racemate), and from the NMR data this same stereochemistry was proposed for a series of congeners. 

The observed diastereoselectivity is consistent with the addition of the aryl group to the intermediate immonium ion, in which approach of the aryl group occurs from the side opposite to the pyrrolidine 2-substituent, as presented in formula XII. 

In a related papCT, high diastereoselectivity with secondary amines was cOTifmned, while the use of chiral primary amines generally gives products with low to moderate diastereoselectivity [17], Factors affecting the efficiency and stereoselectivity of this reaction, in particular of the structure of the boronate ester and amine, were also determined. 

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