Stereochemistry benzyl esters

The synthesis of 252 began with Brown s asymmetric crotylation to aldehyde 261. The resulting homoallyl alcohol was converted benzyl ester 262, which was reduced to give lactol acetate 263. Axial allylation to 263 formed 2,6-trans-tetrahydropyran 264, which was subjected to ozonolysis to give an aldehyde. Addition of alkenylzinc, prepared by hydrozircona-tion of an alkyne 265, to the aldehyde mediated by chiral ligand 266 yielded allyl alcohol 267 with a 5.1 1 diastereoselectivity [110]. The stereochemistry of the major isomer was found, unexpectedly, to be the S-form at Cl7, which rendered the macrolactonization to adopt the Mitsunobu reaction. The iodide 252, prepared from 267 in three steps, reacted with... 

In the synthesis of 183 and C-5/C-6 cis-carbapenems, the acid 178 was obtained by the TBS protection of the azetidinone nitrogen in 176 followed by hydrogenolysis of the product 177 C-5 benzyl ester [118]. Arndt-Eistert homologation was employed for the introduction of methylene fragment and preserved the cis stereochemistry. 

A prototype reaction for preparing benzocyclobutenes is the addition of benzyne to ethylene, and the extended Hiickel MO method has been used to calculate an energy surface for this reaction. The results, whilst not entirely consistent with some experimental evidence, indicate that the reaction is not simple. The preparation of benzocyclobutenyl acetate from benzyne and vinyl acetate is a known reaction but the reduction of this benzylic ester by lithium and ammonia is a newly reported convenient preparation of benzocyclobutene itself. The stereochemistry of the formation of benzocyclobutene by thermolysis of (178) implicates the o-quinodimethane (179) as an intermediate. The stereospecifically labelled ds-compound (178) gave trans-product (180) contaminated by amounts of the ds-compound (181) which varied with experimental conditions. The direct product under the mildest conditions and with low conversion is (180), which presumably isomerizes to (181) under the more... 

Johnson and coworkers have also applied their cycloaddition chemistiy to the synthesis of (+)-virgatusin (Scheme 26) [26]. From enantiomericaUy pure trani-cyclopropane 93, treatment with piperonal 94 in the presence of AICI3 gave 2,5-c -tetrahydrofuran 95. Interesting was that 95 resulted from retention of the stereochemistry on the cyclopropane which, in a similar fashion to the Yang et al. observation (Scheme 19), is a consequence of reversible tetrahydrofuran formation. As a result of this, the benzyl ester stereochemistry on the cyclopropane was what ultimately dictated the C2 and C5 stereocenters. 

The enolates of aziridinecarboxylic esters behave very much like unstabilised organolithiums - they are configurationally stable organolithiums - and as such, not surprisingly react with retention of stereochemistry. Aziridine 9 was proved to react via 10 with retention with benzyl bromide other electrophiles react stereospecifically, presumably with retention.14... 

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