Homoallylic structure

Both the biradical with the cyclopropylmethyl structure, BR2, and that with the other homoallyl structure, BR3, are too short lived for CIDNP generation, the former because of steric strain and the latter because it is a 1,4-biradical of the Paterno-Biichi type, that is, a structure that is known to undergo fast and efficient intersystem crossing without the participation of the nuclear spins. The latter property explains not only the absence of any polarizations from BR3 but also the fact that S-To-type polarizations... 

Steroids which contain a homoallylic structural unit produce an even greater multiplicity of products by both S nI and Sjf2 interactions with azide ion This point is best illustrated by reference to... 

T his work concerns the study of the polymerization of cyclopropane, substituted cyclopropanes, and conjugated cyclopropanes in the presence of cationic and Ziegler-Natta polymerization. The unsaturation of cyclopropane has been described by several workers in the same way as unsaturated compounds. The unsaturation of cyclopropane compounds, which is the basis for the polymerization of these structures, can be explained by the electronic repartition on the three carbon atoms of the ring. Determination of the dipolar moment of chlorocyclopropane has shown that the carbonium ion resulting from the attack of the ring by a carbo cation is stabilized in a homoallylic structure. 

With Et2AlCl/TiCl4, the polymer structures are different, depending on the reaction temperature. At low temperatures, the polymer is a mixture of structures P9a, P9b, P90 and P9d. Infrared spectrography reveals the presence of P9a structures by vibrations at 920 cm"1 and 1240 cm-1. The homoallylic structure P9b is demonstrated by an absorption band at 890 cm-1. This polyene structure is not alone, since characteristic absorptions of trisubstituted double bonds P9d, RiR2C = CHR3, are visible with shoulders at 930 and 835-840 cm-1 for the trans and cis forms, respectively. 

The results of interaction between solutions of ions 524 and nucleophiles also agree with the homoallylic structure of ions but not with the equilibrium of two cations. In reactions with one and the same nucleophile the increase in the electronseeking properties of the aryl group at C is accompanied by an increase in the tricyclic products second, by a parallel increase of stereospecificity in the nucleophilic attack at C — mainly from the endo side despite the steric accessibility of classical benzyl cations with the same skeleton from the exo side e.g. ... 

These facts readily account for the assumption of the homoallylic structure of the cations discussed with increasing -participation of the double bond the possibility of the nucleophilic break-through from the exo side at C decreases and simultaneously an attack at C becomes more probable. 

The solvolyses of cyclopropylcarbinyl and cyclobutyl derivatives often give exactly the same products, in close to the same ratios, as observed by Roberts in 1951. The reaction of certain homoallyl derivatives will also give these products, but homoallyl structures are also very susceptible to Sn2 reactions and will therefore sometimes deviate in the product ratios. A comparison where all three derivatives give the same products in similar ratios is shown in Eq. 11.42. The data indicate that there is likely a common intermediate in all three of these reactions. 

Figure 10 Portion of the structure diagram for the C4H7 system showing only stable structures. The three-fold symmetry accounts for the scrambling of the three methylene carbon atoms via interconversion of the cyclopropylcarbinyl 1 and cyclobutyl 2 cations. The planar homoallyl structure 3 is obtainable from the same transition state for conversion of 1 to 2. The central structure 4 is a relatively high-energy trimethylene methane cation intermediate, incorrectly assigned a structure corresponding to tricyclobutonium ion in a mechanistic model of the scrambling of the methylenic carbons...

In the alkylative cyclization of the 1,6-enyne 372 with vinyl bromide, formation of both the five-membered ring 373 by exn mode carbopalladation and isomerization of the double bonds and the six-membered ring 374 by endo mode carbopalladation are observed[269]. Their ratio depends on the catalytic species. Also, the cyclization of the 1,6-enyne 375 with /i-bromostyrene (376) affords the endo product 377. The exo mode cyclization is commonly observed in many cases, and there are two possible mechanistic explanations for that observed in these examples. One is direct endo mode carbopalladation. The other is the exo mode carbopalladation to give 378 followed by cyclopropana-tion to form 379, and the subsequent cyclopropylcarbinyl-homoallyl rearrangement affords the six-membered ring 380. Careful determination of the E or Z structure of the double bond in the cyclized product 380 is crucial for the mechanistic discussion. 

All the rearranged products derived from (12) and (15) have been rationalized as arising by proton loss or reaction with fluoride ion of the respective homoallylic C-19 cations. The structures of the cations derived from (15) are represented by structures (20) to (24)." ... 

Scheme 1). Introduction of a jt bond into the molecular structure of 1 furnishes homoallylic amine 2 and satisfies the structural prerequisite for an aza-Prins transform.4 Thus, disconnection of the bond between C-2 and C-3 affords intermediate 3 as a viable precursor. In the forward sense, a cation ji-type cyclization, or aza-Prins reaction, could achieve the formation of the C2-C3 bond and complete the assembly of the complex pentacyclic skeleton of the target molecule (1). Reduction of the residual n bond in 2, hydro-genolysis of the benzyl ether, and adjustment of the oxidation state at the side-chain terminus would then complete the synthesis of 1. 

The surprising selectivity in the formation of 4 and 5 is apparently due to thermodynamic control (rapid equilibration via the 1,3-boratropic shift). Structures 4 and 5 are also the most reactive of those that are present at equilibrium, and consequently reactions with aldehydes are very selective. The homoallylic alcohol products are useful intermediates in stereoselective syntheses of trisubstituted butadienes via acid- or base-catalyzed Peterson eliminations. 

Structural rearrangements are not encountered with saturated Grignard reagents, but allylic and homoallylic systems can give products resulting from isomerization. NMR studies indicate that allylmagnesium bromide exists as a CT-bonded structure in which there is rapid equilibration of the two terminal carbons.101 Similarly,... 

The reaction is also applicable to a wide structural variety of 1,3-dienes, as demonstrated by the reaction of 2-furfural-p-anisidine imine (Table 9). All dienes react with the aldimine regioselectively at the diene termini bearing the highest electron densities and provide bis-homoallyl amines with excellent 1,3-syn diastereoselectivity. No C4-adducts are formed in these reactions. 

Synthetic activity associated with the carbonyl-ene reaction is extensive. During the past decade, the trend has been to perform these reactions in the presence of a Lewis acid in an enantioselective fashion. Efforts to find a general catalyst that affords homoallylic alcohols in high yields and enantioselectivities are continual. The synthetic utility of this reaction has been validated by its application to the synthesis of a number of natural products (see Section 10.12.6) and many structurally novel motifs that have found a place in drug discovery vide infra). It is the latter application that has resulted in research efforts aimed at large-scale production of carbonyl-ene adducts. 

In the case of tri-substituted alkenes, the 1,3-syn products are formed in moderate to high diastereoselectivities (Table 21.10, entries 6—12). The stereochemistry of hydrogenation of homoallylic alcohols with a trisubstituted olefin unit is governed by the stereochemistry of the homoallylic hydroxy group, the stereogenic center at the allyl position, and the geometry of the double bond (Scheme 21.4). In entries 8 to 10 of Table 21.10, the product of 1,3-syn structure is formed in more than 90% d.e. with a cationic rhodium catalyst. The stereochemistry of the products in entries 10 to 12 shows that it is the stereogenic center at the allylic position which dictates the sense of asymmetric induction... 

Reactions of aldehydes with complexes 13—17 provide optically active homoallylic alcohols. The enantioselectivities proved to be modest for 13—16 (20—45% ee). In contrast, they are very high (> 94% ee) for the (ansa-bis(indenyl))(r]3-allyl)titanium complex 17 [32], irrespective of the aldehyde structure, but only for the major anti diastereomers, the syn diastereomers exhibiting a lower level of ee (13—46% ee). Complex 17 also gives high chiral induction (> 94% ee) in the reaction with C02 [32], in contrast to complex 12 (R = Me 11 % ee R = H 19% ee) [15]. Although the aforementioned studies of enan-... 

Aqueous ethanolyses of adamantylideneadamantyl halides show Grunwald-Winstein sensitivity parameters (m) of 0.74 ( 0.06), 0.90 ( 0.01), and 0.88 ( 0.03) for the chloride, bromide, and iodide compounds, respectively. All reaction products are formed with retention of both the ring structure and the stereochemistry of the reaction centre. Observed common-ion rate depressions are consistent with a reaction pathway via a free solvated homoallylic carbenium ion. 

Naturally occurring macrocycles are attractive targets for the synthetic organic community, first of all because of their challenging structures. They have aided progress in the systematic approach to analyze and synthesize stereogenic triads by aldol or homoallyl alcohol methods [5, 6]. 

Cross-cyclization of epoxides with homoallylic amines is an easy way to access tetrahydropyran moieties, which form the core structure of many biologically important natural products such as avermectins, aplysiatoxin, oscillatoxins, latrunculins, talaromycins, acutiphycins, and apicularens. Even though many methods are available for the synthesis of this moiety [14—24], its importance and wide applicability demands further methods. 

These observations were applied by Dimitrov, Hesse and coworkers. On stndying ozonization of a series of allylic and homoallylic alcohols prepared from (+) camphor and (—) fenchone, they were able to isolate a certain number of ozonides and to obtain the O NMR spectrum of the diastereomeric mixture of one of them, i.e. derivative 15, whose structure and O NMR chemical shifts (5, ppm) are shown below. 

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