Enecarbamates, cycloadditions with

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

The first example of a chiral-auxiliary-induced [2+2] cycloaddition between 02 and oxazolidinone-functionalized enecarbamates, which proceeds with complete diastereoselectivity as a result of steric repulsions, has been reported to afford 57 <02JACS8814>. The optically active enecarbamates bearing Evans chiral auxiliary were photooxygenated at -35 °C with 5,10,15,20-tetrakis(pentafluorophenyl)porphine (TPFPP) as sensitizer and an 800 W sodium lamp as light source. The dioxetanes 57 were obtained exclusively, but they readily decomposed at room temperature to the expected carbonyl products because of their thermally labile nature. The absolute configuration of the dioxetanes 57 was established by reduction to the corresponding diols with L-methionine. 

Adam W, Bosio SG, Turro NJ (2002) Flighly diastereoselective dioxetane formation in the photooxygenation of enecarbamates with an oxazolidinone chiral auxiliary steric control in the [2+2] cycloaddition of singlet oxygen through conformational alignment. J Am Chem Soc 124 8814-8815... 

On the other hand, Hayashi et al. have reported the highly enantio-selective formal [3 + 3] cycloaddition of a,p-unsaturated aldehydes with ene-carbamates catalysed by diphenylprolinol silyl ether as an organocatalyst. This reaction consisted of four consecutive reactions including an asymmetric ene reaction, an isomerisation from an imine into an enecarbamate, a hydrolysis and a hemiacetal formation in one pot to afford synthetically important chiral piperidine derivatives with excellent enantioselectivities of up to 99% ee, good yields and moderate to good diastereoselectivities, as shown in Scheme 6.22. 

Scheme 6.22 Formal [3 + 3] cycloadditions of a,P-unsaturated aldehydes with enecarbamates.

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. 

More recent mechanistic studies performed by the same authors point towards a stepwise cycloaddition mechanism when enecarbamates are used. Furthermore, the catalyst loading could be decreased to very low levels (0.5 mol%) with retention of excellent diastereo- and enantioselectivity [107]. 

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