3,5-dialkyl substituents

Aldimines (azomethines) C=N—R, r/ 1673-1639 1405-1400 (s) Dialkyl substituents at higher frequency diaryl substituents at lower end of range... 

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

The best enantioselectivity achieved so far was in intramolecular alkoxycarbonylation reactions. For example, using Pd(OAc)2 with a chiral diphosphine ligand on //-substituted allylic alcohols containing dialkyl substituents at the a-position gave good enantioselectivity [105,106]. More recent studies show that it is possible to carbonylate enantioselectively p,/-substituted allylic alcohols [107]. Thus, following the observations achieved with non-chiral Pd(0)(dppb) complexes that fraus-disubslituted lactones can be selectively produced from -substituted allylic alcohols [87], hindered chiral... 

Reaction of iodine with non-conjugated dienes has been applied to the synthesis of cyclic compounds100. Although the reactions of 1,5-hexadiene, 1,6-heptadiene and 1,7-octadiene with I2 in CCI4 gave exclusively products arising from addition to the two double bonds, the introduction of dialkyl substituents into the 4-position of 1,6-heptadiene completely changed the reaction course in favor of cyclization (equation 85). 

Most acetanilides are biodegraded rapidly in soil, but alachlor appears to be degraded by a mechanism different from that for other members of this group of herbicides. The presence of either the 2, 6 -dialkyl substituents, the N-alkoxylmethyl substituent, or both, may preclude enzymatic hydrolysis of the carbonyl or... 

Stabilized ketene 6S. For l, 2 -disubstituted epoxide, species 6S undergoes 6-endo-dig electrocyclization (path b) [24] to form the six-membered ketone 66, ultimately giving naphthol products. l, 2, 2 -Trisubstituted epoxide species 6S undergoes 5-endo-dig cyclization (path a) to give the ketone species 67, finally producing l-alkylidene-2-indanones. The dialkyl substituent of the epoxide enhances the 5-endo-dig cyclization of species 65 via formation of a stable tertiary carbocation 67. We observed similar behavior for the cyclization of (o-styryl)ethynylbenzenes [15, 16]. Formation of 2,4-cyclohexadien-l-one is explicable according to 6-endo-dig cyclization of a ruthenium-stabilized ketene, vhich ultimately afforded the observed products [25]. 

Fig. 15. The stereoisomeric thiourea-platinum complexes shown in (a) display large 4/ptjC couplings for the a-carbon atom in that branch of the X.X-dialkyl substituent which is in a favourable W configuration with respect to the metal atom (thick bonds). The assignment of the (fortuitously overlapping) H and 195Pt NMR signals of all three stereoisomers was feasible by means of indirect detection of these couplings from a XH detected H/(13C)/195Pt correlation spectrum which is shown in (b) together with the one-dimensional and 195Pt NMR projections. Reproduced from Ref. 44 by permission of John Wiley Sons.

This stereochemical control in hex-5-enyl radical cyclizations can be used for the synthesis of highly functionalized cyclopentanes with vicinal trans- or cis-dialkyl-substituents. The synthesis of a versatile prostaglandin intermediate, Corey lactone 12, from the intermediate 6a (Y = OMe) has been described [14]. 

Recently, much effort has been directed to the synthesis of analogs of the biologically active form of vitamin D3, la,25-dihydroxyvitamin D3 [la,25(OH)2D3] as an important new field in medicinal chemistry [50], As a new and efficient route to this class of compounds, the symmetry -assisted enantiospecific synthesis of the A-ring of the vitamin D-hybrid analogs, 19-nor-22-oxa-la,25(OH)2D3 was reported (Scheme 8C.20) [51]. It should be noted that any gem-dialkyl substituents is not necessary to attain a high level of enantioselectivity. 

The above type of reaction is of particular interest when the N,N-dialkyl substituents are part of a ring system7,8 (e.g., 11). It was found of advantage to replace the acetic anhydride by polyphosphoric... 

Quinolizidine alkaloids occur widely in plants, but none with the 1,4-dialkyl substituents of the amphibian alkaloids. Certain Nuphar plant alkaloids, such as deoxynupharidine, are substituted in the 4 position with a 2-furyl moiety and in the 1 position with a methyl group (142). 

In this chapter, we report measurements on a diaryl-substituted polysilane, poly[bis(p-n-butyl)phenylsilane] (PBPNBPS), in which the bulky phenyl groups may provide more substantial steric hindrance to rotational freedom than do the dialkyl substituents. Reported values of the wavelength of maximum electronic absorption are large (395 10 nm) and cannot be attributed solely to electronic substituent effects (4). 

Initial studies focused on lanthanocene-based catalyst systems that proofed to be efficient in the exo-specific cyclization of terminal aminoalkenes to form five-, six-, and seven-membered azacycles (Scheme 2). The reactions are predictably faster for the formation of smaller five-membered rings and in the presence of em-dialkyl substituents [117]. An increasing metal ionic radius and a more open coordination sphere, for example, in onra-lanthanocenes, are also beneficial for higher cyclization rates [103]. A further increase in catalytic activity is observed when sterically more open and more electrophilic CGC 17 (Fig. 15) are applied [118]. 

In the HP-SbP, system the O-alkyl group of alkyl aryl ethers bearing p-alkyl or 2,6-dialkyl substituents rearranges to the meta position of the aromatic nucleus (Scheme 14,18) [45], Diprotonation of para-substituted phenols with I IP-Sbly occurs at the oxygen and meta carbon atoms, and the resulting diprotonated species undergo Friedel-Crafts alkylation with arenes [46],... 

A series of reduction reactions carried out with various microorganisms and cyclic 1,3-diketones has indicated that the stereochemical course of the reaction depends not only on the relative size of the 2,2-dialkyl substituents but also on the microorganism and can often give products of high enantio- and diastercoselectivity. 

M. J. Devos and A. Krief, Tetrahedron Letters, 1978,1845. For similar work, see P. D. Klemmensen and H. Kolind-Andersen, Ger. Often. 2 732 456 [Chem. Abs., 1978, 88, 136 190 formula (III) of this abstract lacks gem-dialkyl substituents]. 

It has been known for a long time that when m-dialkyl substituents are introduced into a polymethylene chain the equilibrium and rate constants for cyclization are frequently increased. There is not a lot of data available for reactions in the gas phase, but the effect of gf m-dimethyl substituents on the cyclization of cyclopentane and cyclohexane are known (Table 14). 

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