Dienyl homoallyl

Scheme 16 Intramolecular reductive homoallylation of a>dienyl aldehydes...

For example, a dienyl aldehyde reductively cyclizes in the presence of an Ni(0)/PPh3 complex and triethylsilane to give homoallylic cyclopropentanol with high regio- and stereoselectivities, while bishomoallylic cyclopropentanol is obtained as major product under the conditions using stoichiometric Ni(0)-diene complexes (Scheme 85). 

Starting from 2,4,6-octatriene and pivaldehyde, the conjugated homoallylic alcohol 8 is obtained as the sole product. Cycloheptatriene-derived complexes react with aldehydes and C02 to afford mixtures of the isomeric 1,3- and 1,4-cycloheptadienyl carbinols or acids, respectively. Interestingly, analogous reactions with methyl chloroformate or dimethyl carbamoyl chloride produce the conjugated dienyl ester 9 or amide 10 as unique products [19,20]. 

HOMOALLYLIC ALCOHOLS Allyltin dilluoroiodide. Dichlorobis(cycIo-penta-dienyl)titanium. Ethylaluminum dichloride. Stannous fluoride. Trimethyl-aluminum-Dichlorobisfcyclopenta-dicnyl)zirconium. 

Allyltributyltin, 10 Vinyltrimethylsilane, 343 Homoallylic ethers Allyltrimethylsilane, 11 Crotyltrimethylsilane, 86 Trityl perchlorate, 339 Homopropargylic alcohols Allenylboronic acid, 36 D imethoxy [ 1 -trimethylsilyl-1,2-buta-dienyljborane, 218 Lithium acetylide, 44 Triisopropoxy[l-(trimethylsilyl)-l,2-buta-dienyl]titanium, 218 Hydroperoxides Oxygen, singlet, 228 Hydroxy acids a-Hydroxy acids... 

It appears that the intermediate primary norbornadien-7-ylmethyl cation did not undergo ring enlargement leading to product arising from a secondary bicyclo[2.2.2]octa-2,5-dienyl cation but, more likely formation of a delocalized homoallyl cation through the double bonds of the norbornadiene system - is responsible for the formation of the tetracyclooctane derivative. Such homoallyl cations have been considered as intermediates in the biosynthesis of terpenes. 

The regioselectivity in palladium-catalyzed alkylations has been attributed to the dynamic behavior of trihapto pentadienyl metal complexes . For example, competing electronic and steric effects influence product formation in dienyl epoxides, but in palladium-catalyzed reactions steric factors were often found to be more important. Thus, alkylation of dienyl epoxide 76 with bulky nucleophiles such as bis(benzenesulfonyl)me-thane in the presence of (Ph3P)4Pd occuned exclusively at the terminal carbon of the dienyl system producing allyl alcohol 77 (equation 39). However, the steric factors could be overcome by electronic effects when one of the terminal vinylic protons was replaced with an electron-withdrawing group. Thus, alkylation of dienyl epoxide 78 affords homoallylic alcohol 79 as the major product (equation 40). 

As shown in racemic series [220], the stereoselectivity of the allylation of chiral 1,3-dioxanes 6.20 depends upon the reaction conditions and upon the nature of the reagent (silane or stannane). The most selective reagent is CH2=CHCH2SnBu3. Most of the reactions have been carried out with trimethyl-allylsilane in the presence of TiC1.4/Ti(0/-Pr)4 under the conditions recommended by Johnson and coworkers [213], After oxidation of the products and treatment with a base, nonracemic homoallylic alcohols are obtained with excellent enantiomeric excesses (Figure 6.58). From CH2=C=CHCH2SiMe3, dienyl alcohols are similarly obtained [1228, 1229] (Figure 6.58). The TiC -catalyzed reactions of dioxanes 6.20 with alkynylsilanes also lead to nonracemic a-alkynyl alcohols [223],... 

Reactions of reagents 1 and 2 with metal-complexed aromatic, propargylic and dienylic aldehydes provides homoallylic alcohol products with improved selectivity compared to their uncomplexed counterparts. The reaction of benzaldehyde chromium tricarbonyl complex 14 with (R,R)-1 followed by oxidative decomplexation provided ( -15 in 90% yield and 83% ee. The (JE)-crotylboration of 14 with (R,R)-2 provided 16 in 90% yield and 92% ee. Reaction of aldehyde 14 with (. -crotylboronate 3, however, provided adduct 17 in only 41% ee. 

If the initial cyclization of trans trans- or cis, /raw -farnesyl pyrophosphates (52-OPP and 51-OPP) occurs upon the terminal (10, 11) double in an anti-Markovnikov orientation, eleven-membered cyclic dienyl carbonium ions are produced. Deprotonation of the trans, trans-intermediate affords humulene (97) while bis-homoallyl- cyclobutyl carbinyl cyclization furnishes, after elimination, caryophyllene (99). Although the all traw -configuration of the three double bonds in humulene would appear to require trans, rra -farnesyl pyrophosphate as precursor, the possibility exists that the immediate predecessor of caryophyllene is cis, /rtz 5-farnesyl (or cisoid nerolidyl) pyrophosphate. 

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