Carbamates O-allylic

Scheme 6.19 Palladium-catalyzed generation of O-allylic carbamates from amines, C02, and allylic chlorides.

Ono, K., Fugami, K., Tanaka, S. and Tamaru, Y. (1994) Palladium catalyzed arylation of IV-alkyl O-allylic carbamates synthesis of cinnamyl alcohols via Heck arylation. Tetrahedron Lett., 35, 4133-6. 

Scheme 40.27 Tertiary amine-catalyzed rearrangement of O-allylic carbamates to allylic amines.

Scheme 40.28 Asymmetric organocatalytic rearrangement of an O-allylic carbamate to an allylic amine with in situ generation of the substrate.

Functionalized allyllithium compounds of type XIII are also homoenolate equivalents [122,130], but in their reaction with electrophiles sometimes it is not possible to control the regioselectivity. These compounds have been prepared mainly by either deprotonation or tin-lithium exchange. Deprotonation of (F)-cinnamyl-N,N-diisopropylcarbamate 155 with n-BuLi in the presence of (-[-sparteine in toluene gave a configurationally stable lithiated O-allyl carbamate (epi-156), which equilibrates at -50 °C to give the (R)-intermediate 156. Whereas the reaction of these compounds with Mel and MeOTs gave the /-attack, however acylation, silylation and stannylation took place at the a-position (Scheme 2.21) [131]. 

The first successful examples for asymmetric deprotonation by alkyllithium/(—)-sparte-ine (11) utilizing O-allyl and O-alkyl carbamates were published in 1989-1990 by Hoppe and coworkers °. Later, in 1991, Kerrick and Beak contributed the application of this method to Ai-Boc-pyrrolidine" (Sections II.D.l). 

Pd(0)/phosphine complexes, or their precursors, in the presence of a suitable co-base, have also been shown to promote, in good yields (66-100%), the formation of allylic carbamates from various primary and secondary aliphatic amines, pressurized C02 and allylic chlorides, in THF, at ambient temperature [87a]. The choice of the added co-base (Base), used for generating the carbamate salt RR NC02 (BaseH)+, was found to be critical for high yields of O-allylic urethanes. The use of a guanidine (CyTMG) or amidine (DBU) base was optimal for this system (see also Section 6.3.1). ft is assumed that this chemistry passes catalytically through a mechanism similar to that illustrated in Scheme 6.19. This involves nucleophilic attack by carbamate anion on a (tt-allyl) palladium species, formed by the oxidative addition of the allylic chloride to a palladium(O) intermediate. 

The reaction of allyl carbamates (189) with activated enones (188) catalysed by (Ph3P)4Pd in THF proceeded smoothly at room temperature to give the corresponding o ,/3-bis-adducts (190) in high yields.235... 

Allyl carbamates 19 are even more versatile, and the lithio derivatives 20 of allyl carbamates are the most important class of homoenolate equivalents.17 Lithiated allyl carbamates react reliably at the y-position with aldehydes and ketones but less regioselectively with alkylating and silylating agents. O-Benzyl carbamates 21 are readily deprotonated and can be quenched with electrophiles.17 20... 

Deprotonation of O-alkyl carbamates may be achieved in an enantioselective manner with s-BuLi-(-)-sparteine, and the most effective of these reactions employ the oxazolidinones 411. The related compounds 412 perform similarly, but have less neat NMR spectra. Enantioselective lithiation of 413, followed by carboxylation and methylation with diazomethane, generates the protected a-hydroxy acid 414 in >95% ee.176 Many other electrophiles perform well in the quench step, but not allylic or benzylic halides, which lead to partial racemisation.177 30... 

High-purity o- and p-methylstyrenes toluenes by this method. Some other w of l,3-dioxep-5-ene, allyl carbamates. ... 

Heteroatom-assisted metallation reactions are very common and especially useful for the selective o-functionalisation of aromatic compounds. More recently, activation of non-aromatic substrates has attracted attention but only a few examples pertinent to protecting groups will be cited here. Kerrick and Beak [Scheme 1.53] showed that pyrrolidine protected with a /err-butoxycarbonyl (Boc) group 53.1 is enantioselectively deprotonated by 5cc-BuLi activated by the alkaloid (-)-Sparteine (533). Enantioselective deprotonations mediated by (-)-Sparteine were first exploited by Hoppe and co-workers for the metallation of saturated and allylic - carbamates. 

A new glycosylation method based on the mild activation of 0-glycosyl N-allyl carbamates with soft electrophiles has been described by Kunz and co-workers [116]. This activation is similar to that of O-glycosyl 4-pentenoyl esters reported by Fraser-Reid [117], with formation of an oxazolidinone instead of a lactone as the leaving group at the anomeric center. 

---