Intramolecular reaction stereoselective cyclization

Two approaches to convergent steroid syntheses are based on the thermal opening of benzocyclobutenes to the o-quinodimethane derivatives (see p. 80 W. Oppolzer, 1978 A) and their stereoselective intramolecular Diels-Alder cyclizations. T, Kametani (1977 B, 1978) obtained (+ )-estradiol in a six-step synthesis. The final Diels-Alder reaction occurred regio- and stereoselectively in almost quantitative yield, presumably because the exo transition state given below is highly favored over the endo state in which rings A and D would stcrically inter-... 

Aqueous hydrofluoric acid dissolved in acetonitrile is a good catalyst for intramolecular Diels-Alder reactions [9] This reagent promotes highly stereoselective cyclizations of different triene esters (equation 8) The use of other acids, such as hydrochloric, acetic, and trifluoroacetic acid, results in complete polymerization of the starting trienes [9] (equation 8)... 

Chelation control of the intramolecular reaction between an allylsilane and an aldehyde or ketone has been carefully investigated. Excellent stereoselectivity was found for cyclization of B-oxo esters using titanium(IV) chloride as the Lewis acid, less good selectivity for cyclization of /l-diketones70. 

Intramolecular reactions can also occur between carbonyl groups and allylic silanes. These reactions frequently show good stereoselectivity. For example, 7 cyclizes primarily to 8 with 4% of 9 as a by-product. The two other possible stereoisomers are not observed.98 The stereoselectivity is attributed to a preference for TS 7A over TS 7B. These are both synclinal structures but differ stereoelectronically. In 7A, the electron flow is approximately anti parallel, whereas in 7B it is skewed. It was suggested that this difference may be the origin of the stereoselectivity. 

The stereoselective intramolecular Henry reactions have been reported by Seebach. The Michael addition of doubly deprotonated acetyl acetaldehyde to l-methylenedioxyphenyl-2-nitroethene followed by subsequent intramolecular nitro-aldol cyclization leads to the diastereomerically pure cyclohexanone derivative, where the nitro and OH groups are cis as shown in Eq. 3.73.114 This reaction is applied to the synthesis of l-desoxy-2-lycorinone as shown in Eq. 3.74.115... 

Recently, Yu and co-workers developed an operationally simple catalytic system based on [RuCl2(/>-cymene)]2 for stereoselective cyclization of a-diazoacetamides by intramolecular carbenoid C-H insertion.192 /3-Lactams were produced in excellent yields and >99% m-stereoselectivity (Equation (53)). The Ru-catalyzed reactions can be performed without the need for slow addition of diazo compounds and inert atmosphere. With a-diazoanilide as a substrate, the carbenoid insertion was directed selectively to an aromatic C-H bond leading to y-lactam formation (Equation (54)). 

The convergence of the nitronate and nitrile oxide cycloadditions has allowed for the direct comparisons of yields and stereoselectivities of the two processes. For intramolecular reactions, the nitronate dipole typically required longer reaction times and/or elevated temperatures (22,98,135), however, the nitronate cycloaddition shows considerably higher diastereoselectivity (Table 2.42). Interestingly, the diastereoselectivity is dependent on the placement of a substituent on the tether. In the case of the silyl nitronate derived from 172, the diastereoselectivity is controlled by the substituent at C(l), while cyclization of the analogous nitrile oxide is governed by the substituent at C(l ) (Scheme 2.10) (124). 

The use of isopropylidine acetals (112) as tethers in the intramolecular Diels-Alder reactions of dienes with alkenes facilitates the formation of civ-fused cycloadducts (113) from an endo transition state (Scheme 41).218 The intramolecular Diels-Alder reaction of 4-[tris-(2-mcthylcthyl)silyl]oxy-2//-thiopyran derivatives with potential dienophiles tethered at C(2), C(3), C(5), and C(6) positions yielded cycloadducts when the dienophiles were activated with a carbomethoxy group.219 By the substitution of a phenylsulfonyl group on the dienophile of 2-benzopyran-3-ones, it is possible to enhance exo addition during intramolecular Diels-Alder cyclizations to yield a predominance of trans-fused hexaphenanthrenes related to natural products.220 The intramolecular Diels-Alder reaction of 2-furfuryl fumarates has been investigated by molecular mechanics (SIBFA)/continuum reaction field computations.221 The intramolecular 4 + 2-photo-cycloaddition of A-benzylcinnamamides (114) in the presence of C(,H6 gives 3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones (115) with high stereoselectivity (Scheme 42).222... 

Once a chiral silane is synthesized, intramolecular reaction such as the Heck reaction and the radical cyclization reaction with transfer of chirality from the silicon center can be pursued. Other reactions of interest include the stereoselective hydrosilylation of aldehydes and ketones. Our method could be used to tether chiral silanes to solid support for uses in solid support... 

Electrophilic substitutions of alkenyl-, aryl-, and alkynylsilanes with heteroatom-stabilized cationic carbon species generated by the action of a Lewis or Brpnsted acid (acyl cation, oxocarbenium ion, etc.) provide powerful methods for carbon-carbon bond formation. Particularly, intramolecular reactions of alkenylsilanes with oxocarbenium and iminium ions are very valuable for stereoselective construction of cyclic ether and amine units.21-23 For example, the BFj OEt -promoted reaction of (E)- and (Z)-alkenylsilanes bearing an acetal moiety in the alkenyl ligand gives 2,6-disubstituted dihydropyrans in a stereospecific manner (Scheme l).23 Arylsilanes also can be utilized for a similar cyclization.24... 

The very fast metal-halogen exchange allows intramolecular cyclization reactions, which are known as Parham cyclizations171. The potential of Parham cyclizations as a useful stereoselective cyclization procedure has proven to be extremely interesting172. Thus, it has been recently demonstrated that iodinated IV-phenethylimides tolerate iodine-lithium exchange, giving rise to the isoquinoline nucleus 304, via a Parham-... 

Intramolecular Marschalk reaction. Ai. intramolecular Marschalk reaction (9, 376) can be used to effect a synthesis of anthracvclinones from anthraquinones. Thus the oi-hydroxy aldehyde 2, formed on saponification of the a-hydroxydichloride 1, on reduction of the quinone group cyclizes in the alkaline medium to the tetracyclic tran.s- and ci/j-diols(3and4)inaboutequal amounts. Cyclization underphase-transfer conditions results in improved yields and, more importantly, can alter the stereoselectivity. Triton B is the most effective catalyst for stereoselective cyclization to the desired natural tran -diol. 

As discussed earlier (Section 6.3.4), acyclic (pentadi-enyl)iron cations are intermediates in reactions involving protonation or Lewis acid activation of iron dienol or dienol acetate complexes. With appropriate functionalization, these cations may also be trapped intramolecularly (Scheme 81). In the case of the formation of tetrahydrofinans (278) and (279), equilibration of the transoid and cisoid cations is observed. However, this equilibration is not observed in the stereoselective cyclization to provide (racemic) oxocene (280). Analogous intramolecular cyclizations of sullur and nitrogen... 

The intramolecular mercury-induced cyclization of enol ethers has proven useful for the dia-stereoselective preparation of disaccharides. The reaction is carried out by treating the enol ethers (e.g., 7) with mercury(ll) trifluoroacetate in tetrahydrofuran for 2 hours at 0 °C, followed by reaction with aqueous potassium chloride at 0 °C for 1 hour. The chloromercury derivatives are obtained in good yield and only one isomer can be detected in the reaction mixture95-96. 

Intramolecular reaction of an allylsilane and an aldehyde was performed to prepare cyclic compounds as illustrated in Eq. (90) [233], which shows a high diastereoselec-tivity. The cyclization of optically active allylsilane proceeds stereoselectively in a manner consistent with the antiS l mechanism (Eq. 91) [234]. Development of a new reagent with a bis-allylsilane moiety effected tandem inter- and intramolecular cyclizations to give cyclic compounds as exemplified in Eqs (92) [235] and (93) [236-238]. In Eq. (92), the double addition product initially formed underwent a pinacol-type rearrangement under the influence of TiCU to give, eventually, the methyl cyclopentyl ketone. Further examples of intramolecular cyclization of allylsilanes are summarized in Table 8. 

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