Enol carbamates

Hydrogenation of N-Acyl Enamides, Enol Esters and Enol Carbamates... 

Scheme 28.15 Enantioselective hydrogenation of enol acetates and enol carbamates.

Since enol carbamates of 4-hydroxyalkanals (or y-hydroxyalkanones) are produced by formation of the C(3)—C(4) bond, we named the process homoaldol reaction 244-246... 

C(l) in 355 is in the oxidation state of an aldehyde or a ketone in 1-alkylated products. The necessary solvolytic attack of the enol carbamate double bond requires substoichio-metric amounts of a catalyst such as mercuric acetate or palladium chloride and one equivalent of acid (e.g. methanesulphonic acid) for binding the liberated diisopropylamine. 

Another approach for the deprotection of highly functionalized enol carbamates is based on hydroboration (equation 96) . 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

In comparison to other vinylic compounds , the vinyl proton in 1-alkenyl carbamates, deprotonation has a very high kinetic acidity . After protection of the 4-hydroxy group in the homoaldol products by silylation, deprotonation (w-BuLi, TMEDA, diethyl ether or THF) of enol carbamate 384 is complete at —78 °C (equation 103), and the resulting vinyUithium 385 can be kept at this temperature without decomposition for several hours. Stannylation , silylation , methoxycarbonylation (with methyl chloroformate) ... 

Scheme 10.1 Selective formation of enol carbamates from secondary amines, CO2, and terminal alkynes.

The electrophilic activation of alkynes towards nucleophilic adducts of C02 with amines allows the catalytic incorporation of the heterocumulene into enol carbamates. 

Similarly, tetrahydropyrans 189, containing the all-cis substitution pattern could be easily synthesized by using the Z-enol carbamate 188, the geometric isomer of 181. Such a compound was readily prepared by applying the modified allyl-metallation protocol reported by Hoppe to the carbamate derivative 187 (Scheme 13.66) [83-85],... 

Monodentate phosphoramidites, in particular (9) and its octahydro analogue, are found to be excellent ligands for the rhodium-catalysed asymmetric hydrogenation of aromatic enol acetates, enol carbamates, and 2-dienol carbamates with up to 98%... 

All syn-Z fi, 4- 3co-methylene tetrahydropyrans 367 can be readily assembled using a Lewis acid promoted Prins reaction between enol carbamates 368 and aldehydes (Equation 153). The presence of an o-methylene group and the labile carbamate in the product allows for simple transformation into tetrahydropyranones and tetrahydropyranols <2000TL7225, 2001TL8685, 2002JOC8744>. 

Lithiated enol carbamates 559 have been reported as complementary acyl anion equivalents of MVL (524) and EVL (525). Non-fluorinated enol carbamates have been prepared by O-carbamoylation of acetaldehyde lithium enolate (80-87% yield) or by quantitative addition of trimethylsilylamides to vinyl chloroformate837. 

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... 

Lithiated di- and monofluorinated enol carbamates 566844 847 and 567848 are fluorine-containing acyl anion equivalents, which allow the synthesis of a-fluorinated ketones. 

SCHEME 26. Intramolecular cleavage of an enol carbamate by a lithium alcoholate118... 

These enol carbamates are stable to acid, but undergo methanolysis in the presence of methanesulfonic acid and Hg(OAc)2 to form y-lactol methyl ethers, with retention of configuration at C, and C4. These products can be oxidized (8, 97) to -y-lactones. The overall process thus provides a route to protected -y-hydroxy aldehydes. 

A distinctive reaction that uses CO2 is the synthesis of enol carbamate, in which carbamic acid derived from CO2 and amines is added to acetylenes, catalyzed by mthenium complexes. 

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