Allylic oxygenated substituents

Iodocyclization to hydroxy tetrahydrofurans. Cyclization of y,8-unsaturated alcohols to tetrahydrofurans (12, 254-256) can be directed by an allylic oxygen substituent, which also can increase the rate of cyclization. Thus derivatives of 4-pentene-l,3-diol undergo iodocyclization mainly to dr-3-hydroxy-2-iodomethylte-trahydrofurans. 

Table 9 Control of Stereochemistry by Allylic Oxygen Substituents (Equation 38)...

Irreversible acetoxyselenenylation of terminal and disubstituted olefins has been achieved on addition of PhSeBr in an acetate-buffered solution. Styrenes afford only Markovnikov adducts, while simple terminal olefins and olefins containing an allylic oxygen substituent (RCO2 or ArO group) furnish 50-80% of the anti-Markovnikov isomer. The product mixture can be isomerized to contain 90-97% of the Markovnikov product by a catalytic amount 6-41%) of BF3.Et20 in CHCI3259. 

The cycloaddition of a nitrile oxide with a chiral allylic ether affords an isoxazoline with selectivity for the pre/-isomer. This selectivity increases with the size of the alkyl substituent and is insensitive to the size of the allyl oxygen substituent. However, allyl alcohols tend to form the / ar/ isomcr preferentially, although the selectivity is often low.427"434 The product of dipolar cycloadditions based on nitrile oxides, the isoxazoline moiety, can be converted into a large variety of functional groups under relatively mild conditions.3 Among other products, the addition can be used to prepare P-hydroxy ketones (Scheme 26.17).435 The isoxazoline moiety can be used to control the relative stereochemistry through chelation control.436,437... 

The high asymmetric induction of these rearrangements is explained by transition states (La) and (Lb) as shown in Scheme 8a. The allylic oxygen substituents adopt an orientation perpendicular to the plane of the double bond to minimize allylic 1,3-repulsion. The propaigylic anion attacks the double bond preferentially anti to these oxygens. 

Diastereoselectivity has been observed in oxymercuration of alkenes having nearby oxygen substituents. Terminal allylic alcohols show a preference for formation of the anti 2,3-diols. 

Allylic stannaries with y-oxygen substituents have been used to build up polyoxy-genated carbon chains. For example, 16 reacts with the stannane 17 to give a high preference for the stereoisomer in which the two oxygen substituents are anti. This stereoselectivity is consistent with chelation control.183... 

Diastereoselective hydrogenations of this type have been reported by Burgess and coworkers [54—59] using chiral-protected and -unprotected allylic and homo allylic alcohols as substrates with their carbene catalyst lr(9). Catalyst control was found to be dominant, but depending on the position and nature of the oxygen substituents, moderate to strong match/ mismatch effects were observed. 

The stereochemical outcome of the cycloaddition to 3-butene-1,2-diol derivatives, cyclic acetals, or to related alkenes that possess an allylic nitrogen substituent such as 4-vinyl-oxazolines or -oxazolidines was also rationalized by this model (162) (Table 6.7). In the latter cases, the A-Boc group instead of the a-oxygen prefers the inside position (Scheme 6.24). 

Steric effects similar to those shown in equations (39) and (40) are found when the substitution pattern leads to tetrahydropyran systems through 6-endo cyclization. Cyclizations of systems with an allylic oxygen and a syn alkene substituent give products rationalized by cyclization through H-in-plane conformations as shown earlier in equations (31) and (32).105 128 Examples with allylic methyl substitution have been reported also.1040... 

Few examples of stereoselectivity in 6-exo cyclizations to piperidine derivatives have been reported. The aminocarbonylation reactions of systems with an allylic hydroxy substituent that are quite stereoselective in cyclization to pyrrolidine systems (equation 106) are nonselective in cyclizations to piperidine systems.237 Cyclizations with mercury(II) acetate also proceed with low selectivity (59-69% cis isomer).237 A series of aminoalditols have been synthesized by aminomercuration of oxygen-substituted 6-(/V-benzylamino)hexenes. The stereochemistry of these cyclizations (equation 112)247 does not appear... 

Iodocyclization of trichloroacetimidates forms trans-1,3-oxazolines with moderate to good stereoselectivity (equation 118).234 Cyclizations of systems with internal double bonds (equation 119) generate 5-exo products only if the double bond has a (Z)-configuration (entries 2 and 3) or an oxygen substituent is attached to the allylic carbon (entries 3 and 4). Wc-153 ,i57b,254c... 

While the process works for a great number of conjugated dienes, a few, such as 1,3-cyclopentadiene and those acyclic dienes that have an oxygen substituent in an allylic position, did not give a chloroacetoxylation product.23 Control of the 1,4-relative stereochemistry and preparation of compounds analogous to the title compounds also work for acyclic dienes,2a>5 This process was used to obtain remote stereocontrol in acyclic systems and applied to the synthesis of a pheromone.5... 

An additional substituent at C-2 does not alter the relative energies of the transition states thus, trisubstituted alkenes are accessible with a high degree of stereoselectivity. Examples arc the synthesis of the natural product nuciferol8 and other allylic oxygenated terpenes9 or 2-phenylthioallyl alcohols10. 

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