Cyclization asymmetric allylation

Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphi-noxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereo- and enantioselective allylation of substituted nitroalkanes has also been reported.9513... 

In intramolecular cyclopropanation, Doyle s catalysts (159) show outstanding capabilities for enantiocontrol in the cyclization of allyl and homoallyl diazoesters to bicyclic y-and <5-lactones, respectively (equations 137 and 138)198 205. The data also reveal that intramolecular cyclopropanation of Z-alkenes is generally more enantioselective than that of E-alkenes in bicyclic y-lactone formation198. Both Rh(II)-MEPY enantiomers are available and, through their use, enantiomeric products are accessible. In a few selected cases, the Pfaltz catalyst 156 also results in high-level enandoselectivity in intramolecular cyclopropanation (equation 139)194. On the other hand, the Aratani catalyst is less effective than the Doyle catalyst (159) or Pfaltz catalyst (156) in asymmetric intramolecular cyclo-propanations201. In addition, the bis-oxazoline-derived copper catalyst 157b shows lower enantioselectivity in the intramolecular cyclopropanation of allyl diazomalonate (equation 140)206. 

Planar chiral phosphaferrocene-oxazolines (379) constitute another family of complexes that are usefiil as ligands in asymmetric catalysis. Preparation of these takes advantage of a modified Friedel-Crafts acylation of (373) and an unusual conversion of the resulting trifluoromethyl ketone into an amide that is then cyclized to an oxazoline. The diastereomeric complexes thus formed are chromatographically separable and are used in a palladium-catalyzed asymmetric allylic substitution. Modification of this complex by using the anion derived from 3,4-dimethyl-2-phenylphosphole gives more... 

Intramolecular asymmetric allylic substitution reactions have been applied to the synthesis of optically active cyclic compounds. The high efficiency of the dihydroxylated ferrocenylphosphine 8b has been shown in the cyclization of 2-butenylene dicarbamates 52 to form optically active 4-vinyl-2-oxazolidones 53, which are... 

One of the exciting developments of the Wacker-type oxidation is the asymmetric synthesis of the reaction. For instance, using a new chiral bis(oxazoline) ligand L = 3,3 -Disubstituted 2,2 -bis(oxazolyl)-l,r-binaphthyls (boxax), a catalytic asymmetric Wacker -type cyclization converted allyl-phenol 55 to dihydrofuran 74 with 67% ee.ss... 

Shibasaki and coworkers [40] also demonstrated the use of soft carbanionic nucleophiles, initially sodium dimethyl malonate, in cascade asymmetric Mizoroki-Heck cyclization- j -allyl trapping sequences. This conversion succeeds with various soft carbanionic nucleophiles to provide functionalized bicyclo[3.3.0]octane derivatives 55 in excellent yields (72-92%) and up to 94% ee (Scheme 16.13). The enantioselectivity of these Mizoroki-Heck reactions is significantly diminished in the absence of NaBr a speculative rationale to account for the effect of the NaBr additive has been advanced [40]. 

Non-racemic epoxide 307, prepared from (+)-epichlorohydrin, was alkeny-lated to give allyl silane 308, which was converted into allyl stannane 309. Corey s chiral boron reagent-mediated asymmetric allylation [120] to aldehyde 311 provided homoallylic alcohol 306 with a 11 1 diastereomer ratio. Intramolecular Sn2 cyclization of the corresponding 7-hydroxy-3-TsO unit derived from 306, followed by hydrolysis of the dithiane, afforded aldehyde 312 (Scheme 66). 

As an alternative, iridium complexes show exciting catalytic activities in various organic transformations for C-C bond formation. Iridium complexes have been known to be effective catalysts for hydrogenation [1—5] and hydrogen transfers [6-27], including in enantioselective synthesis [28-47]. The catalytic activity of iridium complexes also covers a wide range for dehydrogenation [48-54], metathesis [55], hydroamination [56-61], hydrosilylation [62], and hydroalkoxylation reactions [63] and has been employed in alkyne-alkyne and alkyne - alkene cyclizations and allylic substitution reactions [64-114]. In addition, Ir-catalyzed asymmetric 1,3-dipolar cycloaddition of a,P-unsaturated nitriles with nitrone was reported [115]. 

Lucas and Burke reported on aPd(0)-catalyzed asymmetric allylic etherification/ desymmetrization of meso substrates by double cyclizations to the bis-THP subunit of phorboxazoles A and B [51]. Based on Trost and Toste s transition state model for the diphenylphosphino benzoic acid (DPPBA) ligand system [52], Burke rationalized that cyclization of tetraol 69 with (/ ,/ )-DPPBA ligand would effect the double cyclization to give the trans-A ring and cts-B ring in bis-THP 70 (Scheme 24). Under optimized cmiditions, tetraol 69 was converted to 70 in 75 %... 

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