High control of the regiochemistry in reactions of heterosubstituted allylic carbanions via allylic aluminum "ate" complexes

Regioselectivity of the Reactions of Heteroatom-Stabilized Allyl Anions with Electrophiles

“…Replacement of the lithium counterion in 40 by treatment with zinc chloride 34 to give 51 as does CdCl 2 , results in exclusive formation of α-adducts 49 in subsequent reactions with aldehydes and ketones. Similar α-direction is achieved upon replacement of the lithium counterion with triethylaluminum , or via boron “ate” complexes (Scheme ). Another way to achieve α-attack is to use (α-ethoxybutenyl)tributyltin .…”

“…Thioethers show an opposite behavior with respect to the oxygen-substituted allylic anions in that allyl thioether anions 101 are α-alkylated to 98 and react at the γ-terminus with carbonyl compounds to form 99 (Scheme ). High α-selectivity is achieved upon alkylation in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) due to the formation of dissociated ion pairs (in the absence of DABCO: intimate ion pairs). , α-Regioselectivity ( 110 → 111 ) is also enhanced by intramolecular chelation of lithium to the heteroaromatic ring (Scheme ) 16

17

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“…The addition of DABCO to alkyl sulfide anion 99 (Scheme ) favors the formation of dissociated ion pairs and therefore α-selective alkylation to give 100 . , …”

“…While triethylalumium/trialkylboron “ate” complexes produce a mixture of the erythro- and threo-isomers with aldehydes, aluminum compounds give only erythro-products with very high stereoselectivity . To improve the stereoselectivity of the reaction with chiral aldehydes Bu 3 SnCl/BF 3 could be used which gives either predominantly or exclusively to the α-syn-product …”

Part I Towards a total synthesis of papulacandin D Part II Towards a total synthesis of amphidinolide X

“…97,98 The general scheme by which Lewis base-promoted allylation gives rise to improved diastereoselectivity can be attributed to the more rigid, closed transition state as shown in Scheme 4.5. [97][98][99][100][101] This is in contrast to Lewis acid-catalyzed allylation, which goes through a more flexible open transition state mechanism and hence gives poorer diastereocontrol.…”