Unsaturated esters, alcohols from selective

The construction of the alkenyl side chain and the control of the C9, CIO and Cll stereogenic centers was achieved from (5)-(+)-methyl 3-hydroxy-2-methylpropionate 1. (Scheme 21) This compound was transformed to aldehyde 99 in three steps. Bis(2,2,2)trifluoroethyl)[(methoxycabonyl)methyl]-phosphonate [23] was employed for the selective formation of the cA-a, 3-unsaturated ester 100. From this Z-unsaturated ester 100, the three consecutive asymmetric units were constructed via epoxide 101 (m-CPBA), which was selectively opened by lithium dimethylcuprate to produce 102. After deprotection-protection, the alcohol 102 was converted to the phosphonium iodide 103 via a tosylate intermediate(Scheme 21). 

S)-5-Hydroxymethyl-2(5H)-furanone (8), derived from D-ribono-lactone, was used as an intermediate in a synthesis of the 6a-carbocycline analogue (9), with a Pauson-Khand reaction being used to build up the ring system from enyne (10) (Scheme 4). When alcohol (8) underwent Diels-Alder reaction with cyclopentadiene, either the (2S, 3R)-isomer (11) or its enantiomer could be produced by appropriate manipulation of the initial adduct face-selective Lewis-acid-catalysed Diels-Alder reactions of a,iS-unsaturated esters derived from D-mannitol could be used to produce the (R,R)-isomer (12) or its enantiomer.This chemistry was extended to give syntheses of (+)-0-santalene (13) and its enantiomer from (8) the starting material was prepared as described in Vol. 20 p.260, but a number of other papers dealing with routes to this intermediate are mentioned in the next section. 

The preparation of 1 started with the addition of lithiated 4 to the enantiomcrically-pure epoxide 5, which was prepared from the racemate using the Jacobsen protocol. Reduction followed by selective protection of the primary alcohol gave the monosilyl ether, which was further protected with MOM chloride to give 7. Pd-mediated oxidation to the methyl ketone followed by condensation with the Horner-Emmons reagent gave the unsaturated ester 8 as an inconsequential mixture of geometric isomers. Oxidation then set the stage for the crucial cyclization. 

This reagent readily reduces esters in toluene, hexanes, THF or CH2CI2 solutions, etc. The reaction is especially useful for the preparation of allylic alcohols from a,3-unsaturated esters. 3-Keto esters have been selectively reduced to 3-hydroxy ketones via ketone enolate formation and AIH3 reduction. 

The ate complex LiAlHBu"Bu 2 is prepared from DIBAL-H and n-butyllithium in either THF or toluene-hexane. This reagent is more effective for selective 1,2-reduction of enones to the corresponding allylic alcohol than is DIBAL-H alone. The reagent also reduces esters, lactones and acid chlorides to the corresponding alcohols, and epoxides to the respective alcohols. a,p-Unsaturated ketones derived from dehydration of aldol products from I-(arylthio)cyclopropanecarbaldehydes and ketones were selectively reduced by this ate complex or by DIBAL-H itself, yielding the allylic alcohols with minor... 

An alternate procedure involves the precipitation of mixed copper and chromium hydroxides from a solution of chromium nitrate and copper nitrate by the addition of an sodium bicarbonate and then heating the precipitate to 300°-500°C.30 In this procedure, the copperchromium ratio can be varied over a wide range. A ratio between four and eight was optimum for use in the hydrogenation of esters to alcohols (Eqn. 13.6).30 A related Zn-CrO catalyst prepared by the decomposition of precipitated zinc-copper hydroxides was effective in the hydrogenation of unsaturated esters to unsaturated alcohols (Eqn. 13.7). 0 The presence of a small amount of alumina increased catalyst activity and selectivity. Some of these catalysts, however, tend to become colloidal on use so they can present separation problems. O... 

JV-Phenyl- or A-methyl-C-phenyl nitrones with (/ )-4-vinylsulfmyltoluene react with complete stereoselection in a process involving only the. Si-face of the alkene88. Cycloaddition of 4,5-dihydro-3//-pyrrole 1-oxide to unsaturated esters of chiral alcohols derived from camphor occurs with poor enantiofacial selection on the nitrone, but high diastereofacial preference on the alkene89. 

This reaction was first reported by Johnson et al. in 1970. It is a highly stereoselective synthesis of y,5-unsaturated esters from the reaction between allylic alcohols and an orthoester in the presence of a trace amount of weak acid, such as propionic acid. Because this reaction is the modification or variant of the Claisen Rearrangement, it is often referred to as the Johnson orthoester Claisen rearrangement. Occasionally, this reaction is also known as the Claisen-Johnson orthoester rearrangement, " or Johnson orthoester protocol. This reaction involves the formation of mixed orthoester from allyl alcohol and the added orthoester, which loses an alcoholic component to form a ketene acetal then migrates to unsaturated carbonyl compounds via the Claisen Rearmagement with high syn selectivity. Posner further extended this reaction to use sulfonyl orthoester. Overall, this reaction has been applied to the synthesis of a variety of complicated natural products, such as squalenes. ... 

The commercial exploitation of sperm oil has led to the depletion of whale populations and is banned in some countries. Attention has, therefore, turned to the jojoba plant whose oil also consists of wax esters. Most fatty chemicals obtained from natural sources have chain lengths of Cig-Cig. The limited availability of compounds with 12-14 carbon atoms, which are important in surfactants, was one of the driving forces behind the development of petrochemical processes for the production of fatty alcohols. Higher alcohols, such as C20-C22 alcohols, can be produced from rapeseed oils rich in erucic acid and fish oils. Unsaturated fatty alcohols may be manufactured in the presence of selective catalysts. 

Other examples were also described by Schobert et al ot,P-Unsaturated esters can be prepared in a one-pot process from a variety of alcohols. In the first step, the alcohols are oxidized selectively by PhI(OAc)2/2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to aldehydes that are then subjected to Wittig olefination with ethox-ycarbonylmethylene- or a P-oxomethylenephosphorane (Scheme 94). Substituent Y may include unsaturated, hydroxyalkyl and aryl groups, as well as epoxy-, sugar and pinane-based substituents. 

---