Allyl carbamates, substitution

Wlien an allylic carbamate b syn substitution occurs [ 54], For diliydroxy-16-ene-vitaniin D 165 9.16) [55], In route A, tlie CD side cli S 2 syn substitution of allylic... 

Cuprates react rapidly witli allylic halides for acetates) [17, 23], propargyl halides for acetates) [ 106-108], and vinyloxiraties, often witli S 2 regioselectivity iSclieme 10.9) [17]. Hie reaction takes place witli emit stereodieniistry iwitli respect to tlie leaving group), while syn substitution occurs when an allylic carbamate is employed as tlie substrate [109]. 

Remarkable y-syn-substitution preference is observed in the reaction of allylic carbamates with organocuprates. 

Allyl carbamates also can serve as amino-protecting groups. The allyloxy group is removed by Pd-catalyzed reduction or nucleophilic substitution. These reactions involve formation of the carbamic acid by oxidative addition to the palladium. The allyl-palladium species is reductively cleaved by stannanes,221 phenylsilane,222 formic acid,223 and NaBH4,224 which convert the allyl group to propene. Reagents... 

Scheme 6.38. Influence of reagent and alkene geomet7 on allylic substitution of y-silyl-substituted allylic carbamates 181 (Ts = para-toluenesulfonyl, NMP = N-methylpyrrolidinone).

Scheme 6.39. Interpretation of the results of all of y-silyl-substituted allylic carbamates.

Scheme 8.4. Employment of allylic carbamates 1 in cop per-mediated asymmetric substitution.

Allylic substitution with free carbamate nucleophiles was not accomplished until advanced catalyst precursors were developed. However, products from substitution with carbamate nucleophiles were generated by a decarboxylative allylation. In this process, the imidodicarbonate was shown to undergo decarboxylation to form the carbamate nucleophile, and reaction of the resulting carbamate with the 7i-allyliridium intermediate formed branched allylic carbamate products (Scheme 17) [92, 95]. 

Lithiated allyl carbamates stereochemistry of electrophilic substitution... 

Allyl esters, carbonates, and carbamates readily undergo C-O bond cleavage upon reaction with palladium(O) to yield allyl palladium(II) complexes. These complexes are electrophilic and can react with nucleophiles to form products of allylic nucleophilic substitution. Linkers based on this reaction have been designed, which are cleavable by treatment with catalytic amounts of palladium complexes [165,166], For the immobilization of carboxylic acids, support-bound allyl alcohols have proven suitable (Figure 3.12, Table 3.7). 

The kinetically controlled deprotonation of allylic carbamate esters (29) by n-BuLi-(—)-sparteine has preferentially removed the pro-S proton, leading to the lithium intermediate (S)-(30) (Scheme 14).85 Trapping experiments with chlorotrimethylsilane has afforded the a-substitution products, with R-configuration. 

The substituted A,A-diisopropyl enol carbamates 563 were prepared by reaction of metallated allylic carbamates with aldehydes839-843. The stereoselective deprotonation of compound 563 can be performed with f-BuLi841 or n-BuLi-TMEDA839 in THF at —70 to —85 °C to give the lithio derivatives 564 (Scheme 153). These intermediates reacted with... 

Despite the high synthetic value of 1,2-amino alcohols, which are precursors of several therapeutically important sugar fragments, such as daunosamine, vancosamine and ristosamine, the osmylation of allylic amines or their derivatives often occurs with disappointing stereochemical results40 41. (Z)-Substituted allylic carbamates, however, show moderate syn selectivity105. A successful example is illustrated in the synthesis of l-benzyl-3,4-bis(benzyloxy)-5-(l,2-dihy-droxyethyl)pyrrolidine42. 

Enantioenriched a-carbamoyloxy allylic stannanes can be prepared by lithiation of allylic carbamates in the presence of (-)-sparteine (Eq. 42) [62]. Ilie resulting lithiated sparteine complex reacts with BuaSnLi at the a-position to afford the substitution product. The crotyl derivative of 80% ee is thus prepared. This stannane undergoes thermal addition to benzaldehyde at 160 °C to afford the anti- S) adduct of 79% ee in 79% yield. 

Tab. 6.7. Results of allylic substitution of y-si ly I-substituted allylic carbamates 181 with organocopper reagents.

The addition of heteroatom-substituted allylic anions to aldehydes has found particular interest as it represents a homoaldoi reaction. Metalated allyl carbamates seem to be the most valuable reagents developed for this purpose as they are prepared easily and exhibit high y-selectivity and dia-stereoselectivity, at best in their titanated form (Scheme 79). ... 

This methodology was applied for the preparation of tetrapeptides using the selective removal of an allyl carbamate in the presence of a substituted allylic ester [33]. 

Nucleophiles for the substitution can be generated in situ. For example, a synthesis of allylic carbamates and carbonates is conducted using amines and alcohols under CO2. Since disilanes are cleaved in the presence of Pd(0) complexes, their use in the synthesis of allylsilanes is feasible. 

According to Eq. (99), stereochemically homogeneous 3-carbonyl-substituted tetrahydrofurans are constructed in a brick-box system by sequential ho-moaldol and aldol reaction. The metallated allyl carbamate serves as an equivalent for the chiral dianion A, which accepts two different aldehydes B and C in a highly controlled manner [226]. 

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