Hydroxy masked

Another approach to the synthesis of 8-lactones from 8-hydroxy masked acids has been demonstrated by Khan and Paterson [40] (Scheme 4). ZnBr2-catalyzed phenylthioaUcylation of ketene bis(trimethylsilyl)acetals 26 with appropriate a-chlorosulphides 27 afforded the corresponding alkylated product 28, which on hydrolysis underwent lactonization to provide the 8-lactone 29. 

In Entry 5, the carbanion-stabilizing ability of the sulfonyl group enables lithiation and is then reductively removed after alkylation. The reagent in Entry 6 is prepared by dilithiation of allyl hydrosulfide using n-bulyl lithium. After nucleophilic addition and S-alkylation, a masked aldehyde is present in the form of a vinyl thioether. Entry 7 uses the epoxidation of a vinyl silane to form a 7-hydroxy aldehyde masked as a cyclic acetal. Entries 8 and 9 use nucleophilic cuprate reagents to introduce alkyl groups containing aldehydes masked as acetals. 

Preparation of the requisite side chain starts by alkylation of ethyl acetoacetate with 1,3-dibromopen-tane the initially formed bromoketone (shown as the enol 97) undergoes O-alkylation under the reaction conditions to give the dihydropyran 98. Reaction of that masked hydroxy ketone derivative with hydrogen... 

Later, Patterson and Hibbert (S3) stated that ultraviolet absorption of lignin corresponded with the absorptions of hydroxy derivatives of 4-(4,hydroxy-3,methoxyphenyl)-l-propanone and l-(4,hydroxy-3, 5,dimethoxyphenyl)-4-propanone. These authors, therefore, assumed the presence in lignin of a carbonyl group, or of an ethylenic double bond conjugated with an aromatic nucleus. They ascribed the 280 mp band to m-position freedom in the benzene ring and the 310 mp baud to a masked carbonyl group in conjugation with a benzene nucleus. 

Increasing interest is expressed in diastereoselective addition of organometallic reagents to the ON bond of chiral imines or their derivatives, as well as chiral catalyst-facilitated enantioselective addition of nucleophiles to pro-chiral imines.98 The imines frequently selected for investigation include N-masked imines such as oxime ethers, sulfenimines, and /V-trimcthylsilylimines (150-153). A variety of chiral modifiers, including chiral boron compounds, chiral diols, chiral hydroxy acids, A-sull onyl amino acids, and /V-sulfonyl amido alcohols 141-149, have been evaluated for their efficiency in enantioselective allylboration reactions.680... 

Recently, a chemoenzimatic catalized Henry reaction has been reported by El Blidi et al.53 Nitroaldol cyclization between the masked 3-hydroxy-4-nitrobutyraldehyde 72 and dihydroxyacetone phosphate (DHAP) 73, catalyzed by fructose-1,6-biphosphate aldolase (RAMA), afforded the nitro-cyclohexane 74 (Scheme 24). 

The cycloaddition of allenyl sulfoxide 135 and cydopentadiene occurred at room temperature, giving the single adduct 136. The initially formed allylic sulfoxide underwent a rapid [2,3]-sigmatropic rearrangement. Treatment of 136 with trimethyl phosphite furnished alcohol 137. It should be noted that the reaction of methyl 4-hydroxy-2-butynoate with cydopentadiene failed to give 137. Thus, the allene 135 is considered as a masked and more reactive alkyne equivalent. 

Two consecutive enolate alkylations were utilized to generate the quaternary carbon atom (Scheme 38). Alcohol 238 was transformed into the protected hydroxy enone 244. Regioselective deprotonation at the a-position of the ketone 244 led to a cross-conjugated enolate that was alkylated with the allylic iodide 245. The vinyl silyl moiety in 245 represents a masked keto group [127]. The choice of the TBS protecting group for the hydroxyl group at of 244 was crucial in order to prevent the deprotonation at the y-posi-... 

The lithiation of y-chloro acetal 175 with lithium and a catalytic amount of naphthalene (4%) allowed the preparation of the intermediate 176, which can be considered as a masked lithium homoenolate, and was used for the preparation of the hydroxy ketone 179 through the hydroxy acetal 177 and dithiane 178 using known chemistry (Scheme 62)" . 

Electron-poor allylic alcohols, studied by Adam and coworkers, were found to react r/treo-selectively, regardless of whether the hydroxy functionality is free or masked by alkyl or silyl groups (Scheme 35). This diastereoselectivity, for which the nature of the solvent had no significant influence, was attributed to stereoelectronic effects rather than intramolecular hydrogen bonding or steric strain. 

An intramolecular [3 + 2] nitrone cycloaddition reaction has been employed in the chiral assembly of L-acosamine and L-daunosamine (81JA3956). Heating the masked aldehyde (503) with the oxalate salt of (-)-(S)-7V-hydroxy-a-methylbenzenemethanamine generated a nitrone which underwent intramolecular cycloaddition to give an 82 18 mixture of diastereomers (504) and (505). The N—O bond of the major isomer was cleaved with zinc... 

A. Dondoni, G. Fantin, M. Fogagnolo, A. Medici, and P. Pedrini, Hydroxy group inversion in thiazole polyols by an oxidation-reduction sequence. An entry to syn 1,2-diol fragments in masked carbohydrates, J. Org. Chem. 54 702 (1989). 

---