Enecarbamates, arylation

The preparation of 3-pyrroline was recently reported in Organic Syntheses The protected form, N-Boc-3-pyrroline, has been used effectively in Heck arylations for the preparation of various 4-aryl endocyclic enecarbamates. It has also been used in the preparation of regioisomeric 3-hydroxyisoxazolinyl prolines which are medicinally active. The catalytic asymmetric hydroformylation of N-Boc-pyrroline has been investigated. N-Boc-pyrroline has also been utilized in the synthesis of various antibacterial compounds. ... 

An inverse electron demand aza D A reaction of electron rich alkenes with N aryl imines as 2 azadiene (Povarov reaction) provides tetrahydroquinolines. Reactions catalyzed by chiral phosphoric acids yielded different absolute ste reochemical outcomes when ethyl vinyl efher and enecarbamate are employed as electron rich alkenes, although chiral phosphoric acids have the same axial chirality in both cases (see Scheme 3.26). 

N-acyl imines [144]. A slightly modified chiral Br0nsted acid 185 was found to catalytically induce addition of indoles to N-Boc-protected enecarbamates ISK) in high yields and enantioselectivities (Scheme 8.51) [145]. In a related study, Zhou demonstrated the use of a-aryl enamides to obtain optically enriched tertiary amine products [146]. 

Zhu and co-workers [77] have successfully developed the first organocatalytic enantioselective three-component Povarov reaction for the efficient synthesis of enantiomerically enriched (2,4-cis)-4-amino-2-aryl(alkyl)-tetrahydroquinolines. To illustrate the power of this novel catalytic enantioselective three-component Povarov reaction, they applied this methodology to the short and efficient synthesis of torcetrapib (188), a potent cholesteryl ester transfer protein (CETP) inhibitor (Scheme 17.31). Reaction of 4-trifluoromethylaniline 184, propionaldehyde 18, and enecarbamate 185 using phosphoric acid catalyst 186 afforded tetrahydroquino-line 187 in 57% yield with 93% ee. Ethoxycarbonylation, deprotection/acylation, and benzylation provided torcetrapib (188) in four steps with 32% overall yield. 

The catalytic asymmetric /-selective Diels-Alder annulation of a, -unsaturated /-butyrolactams with enones provided a synthesis of, y-functionalized bridged bi-or tri-cyclic dihydropyranopyrrolidin-2-ones in one step (up to 98% yield, >20 1 dr, and 99% ee) The inverse-electron-demand aza-Diels-Alder cycloaddition 0 of A-aryl-a,/0-unsaturated ketimines with enecarbamates in the presence of chiral bifunctional phosphoric acids produced 4,5,6-trisubstituted 1,4,5,6-tetrahydropyridines having three contiguous stereogenic centres in up to 84% yield, 95 5 dr, and 95% 5-Alkenylthiazoles react as in-out dienes with e-poor dienophiles in polar 44-2- 0 cycloaddition reactions. The cycloadditions are site selective. The mechanism is thought to lie between a concerted but highly asynchronous process and a stepwise process. 

The [2+2] cycloaddition reaction of 4-aryl endocyclic enecarbamates 194 with 2-chloroethyIketene, generated in situ, affords exclusively cndo-(2-chloroethyl)-cyclobutanones 195. ... 

We reported our first results with the Heck-Matsuda reaction in the late 1990s for the arylation of chiral, nonracemic, endocyclic enecarbamates 9 to produce the aryl pyrroUdines 8. These interesting intermediates were further applied in the total synthesis of several fully substituted A -heterocycles with pharmacologically active compounds such as the antibiotic and hypertensive pyrrolidine alkaloid codonopsinine 6, and the antitrypanosomal Schramm s C-azanucleoside 7 (Scheme 4). ... 

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