Allylic derivatives diastereoselectivity

Oxygenated allylic derivatives of (Ipc)2BH also show excellent diastereoselectivity. 

SCHEME 46. Diastereoselective hydroboration of allylic derivatives and B/Zn exchange... 

The unique phenomena observed for allyltitanium ate complexes appear to be restricted to allyl groups. Reversal of chemoselectivity in methylation reactions has not been observed to date. Ate complexes of the type 122 and 123 react much faster with aldehydes than with ketones 90>, but the reactions are not as clean as with the allyl derivatives. Aromatic nitro and cyano groups are tolerated. In certain situations reversal of diastereoselectivity is possible using 123 (Sect. D.II). Ate complexes prepared from poly-titanates17) are also aldehyde selective.77 ... 

Nucleophiles such as enolates or substituted allylmetal compounds are known to react with prochiral aldehydes and ketones to form mixtures of threo or erythro adducts. In case of aldehydes, high degrees of diastereoselection have been achieved 48 91,108). In the following three Sections, reactions of titanium and zirconium enolates as well as allyl derivatives are presented. 

Thus ailylboration of carbonyl compounds with B-allylic derivatives of 9-BBN, followed by transesterification with ethanolamine, provides a simple, convenient method for the synthesis of homoallylic alcohols It has been reported that enantiomerically enriched allylic boranes provide homoallylic alcohols with high degree of enantio- and diastereoselectivity. The scheme is illustrated in Eqs. 48 and 49... 

Stereoselective Diels-Alder reactions have been reported in several cases. Enantioselective Diels-Alder reactions of l-phenoxycarbonyl-l,2-dihydropyridine with 1-alkylated acryloyl-pyrazolidin-3-ones using a chiral cationic palla-dium-phophinooxazolidine catalyst afforded chiral isoquinuclidines with excellent enantioselectivity <2005TL5677>. Bismuth(lll) chloride-mediated diasteroselective intramolecular [4-f2] cycloaddition reactions of A-allyl derivatives of pyrazole aldehydes led to fused sulfur-containing pyrazole heterocycles <2003SC3063>. A highly diastereoselective intramolecular hetero-Diels-Alder approach toward tetracyclic pyrazoles from 5-(3-methyl-2-butenylthio)-3-methyl-l-phenyl-4-pyrazolecarboxaldehyde has been reported <1997SL1155>. 

A new efficient and stereoselective synthesis of furo[2, 3 5,6]pyrano[4,3-i)]quinoline derivatives and has been achieved by intramolecular hetero Diels-Alder reactions of aldimines generated in situ from aromatic amines and the O-allyl derivative of the chiral sugar derived aldehyde in acetonitrile in the presence of a catalytic amount (10 mol%) of BiCl3. The products are formed with extremely high (>95%) trans-selectivity in good to excellent yields (Equation 20) [38a]. The authors have also carried out the diastereoselective synthesis of new hexahydropyrazolo[4, 3 5,6]thiopyrano[4,3-t>] in the similar conditions [38b]. 

Aside from the simple allylation and crotylation, other substituted allylic derivatives have been investigated as well. Thus, the vinyl bromide 32 was reported by Maikov and Kocovsky to produce bromohydrin 33 in modest enantioselectivity and excellent diastereoselectivity. BINAP (30) proved to be the best catalyst (Scheme 15.5), whereas other compounds, including METHOX (22) and QUINOX (23), gave inferior results. The bromohydrins thus formed were converted into the corresponding epoxides 34 upon treatment with NaH and other Bronsted bases [14f]. 

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

The cyclic 2,4-dienoate 184, formed by the Pd-catalyzed cyclization of the 1,6-enyne 183, reacted with 154 to form the azulene derivative 185[118], The 3-methylenepyrrolidine 188 is formed by the reaction of the Zn reagent 186 with the chiral imine 187 with high diastereomeric excess. The structure of the allylic ethers is important for obtaining high diastereoselectivity[l 19],... 

Removal of the unsaturated side-chain appendage from C-8 in 22 provides diol lactone 23 and allylic bromide 24 as potential precursors. In the synthetic direction, a diastereoselective alkylation of a hydroxyl-protected lactone enolate derived from 23 with allylic bromide 24 could accomplish the assembly of 22, an intermediate that possesses all of the carbon atoms of PGF2o- It was anticipated that preexisting asymmetry in the lactone enolate would induce the... 

Stockman has reported the preparation of alkyl-, aryl-, and vinyl-disubstituted aziridines with good diastereoselectivities and in good yields through treatment of tert-butylsulfmylimines with the ylide 119, derived from S-allyl tetrahydrothio-phenium bromide (Scheme 1.39) [64]. A range of substrates were tolerated, including heterocyclic, aromatic, and aliphatic substrates (Table 1.16). 

Freparatively useful induced diastereoselectivities have been reported mainly for 1,1-di-substituted allyllithium derivatives which bear carbanion-stabilizing substituents. l-[Methyl-thio-l-(trimethylsilyl)-2-propenyl]lithium106 and the appropriate 1-phenylthio107 derivative, generated from the allylic sulfide with sec-butyllithium, in the reaction with tetrahydropyranyl-protected pregnolone, furnish a single diastereomer. 

Only few allyltitanium reagents bearing a removable chiral auxiliary at the allylic residue are known. The outstanding example is a metalated 1-alkyl-2-imidazolinone14, derived from (—)-ephedrine, representing a valuable homoenolate reagent. After deprotonation by butyllithium, metal exchange with chlorotris(diethylamino)titanium, and aldehyde or ketone addition, the homoaldol adducts are formed with 94 to 98% diastereoselectivity. 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

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