Homoallylic carbonates

Another example is the iodocyclization of homoallylic hydroxylamines derived from D-glyceraldehyde461. Enhanced diastereselectivity is observed in the iodocyclization of the following homoallylic carbonate in equation 56462. 

The acyl moiety of capsaicin contains only one functional group, a double bond that is redundant for activity. On the contrary, the presence of the double bond is critical for the activity of olvanil, and interesting structure-activity relationships were discovered for this compound. Thus, an analog hydroxylated on the homoallylic carbon is easily available from the amidation of commercial and cheap ricinoleic... 

Substituents occupying sites more remote from the alkenic center than the allylic position may also influence the direction of attack on an alkene with diastereotopic Tr-faces. Thus, cyclic alkenes with a sul-foximine group attached to an exocyclic homoallylic carbon atom and a hydroxy group at the allylic position, for example (Iti) in equation (5), undergo syn hydroxylation with a very high degree of dia-stereoselectivity, diol (17) being produced as the sole diastereoisomer. ... 

Other examples of the iodocyclization of homoallylic carbonates include the following ... 

On the other hand, when iodine monobromide in toluene at low temperature was employed to induce the cyclization of secondary rm-butyl homoallyl carbonates, very high 1,3-asymmetric induction was observed, leading mainly to the ris-isomer11, l2. 

Other Metal Catalysts Palladium catalysts are most widely used for C—N bond formation, particularly in Buchwald-Hartwig coupling reactions [48], Halides or pseudohalides are generally used. Recently, direct aminationof C— H bonds has been developed with palladium catalysts. Pd(0)-catalyzed diamination of terminal olefins at allylic and homoallylic carbons takes place via formal sp3 C—H activation under solvent-free conditions [49]. More recently, an asymmetric version of allylic and homoallylic diamination has been successfully achieved using di-tert-butyldiaziridi-none as the nitrogen source (Equation 11.21) [50]. 

Diastereoselective cyclization of homoallyl carbonates The conversion of the carbonate I to 2 has traditionally been effected with l2 in CHjCN at —20°. It can be carried out with higher selectivity with IBr in toluene, particularly at temperatures of... 

The generally higher selectivity of IBr was observed with a number of other homoallylic carbonates. 

Cyclization. It is sometimes advantageous to use this reagent for the halogen-induced alkene functionalization with participation of an internal nucleophile. At low temperatures homoallylic carbonates react with enhanced diastereoselectivity. 

Otherwise, the leaving group may be in the homoallylic situation either in the glycal itself or on a branch carbon atom in both situations the double-bond electrons of the glycal bond to the homoallylic carbon atom to give products with cyclopropane rings fused to furanoid or pyranoid rings, respectively. In this way compounds 76 [105] and 77 [106] are converted to cyclopropa-furanoid and -pyranoid nucleosides (Schemes 25 and 26, respectively). 

Fig. 4 Charge distribution and structures of the cyclohexadi-ene ring at the TS (color code corresponds to the atonric charges, from blue negative to ted positive). The aUyl attack conformations pre-polarize the aUyl carbon from —0.08 a.u. to about —0.17 a.ir., regardless of the attack orientation (Re or Si face). On the other hand, the conformations corresponding to the homoallyl attack prepolarize the homoallyl carbon from —0.24 a.u. to about —0.32. Relative energies of the F6 molecules are shown in parenthesis (kcal/mol, red) compared to the optimized geometry, and arrows indicate the boron attack...

Duan, J. J. W. Smith, A. B. 1993. Iodine monobromide (IBr) at low temperature Enhanced diastereose-lectivity in electrophilic cyclizations of homoallylic carbonates. J. Org. Chem. 58 3703-3711. 

In a similar way, racemic 3-substituted cyclohexanones 121 were reacted with the anion of the chiral phosphoamidate reagent 18a to give two isomeric optically active alkenes, that is (S, ) and (i , Z)-alkenes possessing the opposite absolute configuration at the homoallylic carbon centers, with high enantiomeric excess [55]. 

General procedure Iodine monobromide (l.OM in DCM, 1.8 equiv) is added to a solution of the homoallylic carbonate 25 (1 equiv) in toluene (0.09 M) at —85°C. After 11 hours, the mixture is warmed to room temperature and quenched with an aqueous solution containing 30% Na2S203 and 5% NaHC03. After extraction, the crude is purified by column chromatography on silica gel to provide 26. 

---