Reactivity of Substituents Attached to the Ring Carbon Atoms

6 Reactivity of Substituents Attached to the Ring Carbon Atoms 

The azetidine ring is stable during a variety of transformations of the functional groups present on the ring carbon(s). The reaction of l-ethoxycarbonyl-3-(bromomethyl)-3-chloroazetidines 85 with l,8-diazobicyclo[5.4.0]undec-7-ene (DBU), 

1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide, HCI, Et3N, (L)-AIa-OMe HCI, CH2CI2 

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Reactivity of substituent attached to the ring carbon atom... 

The boron atom dominates the reactivity of the boracyclic compounds because of its inherent Lewis acidity. Consequently, there have been very few reports on the reactivity of substituents attached to the ring carbon atoms in the five-membered boronated cyclic systems. Singaram and co-workers developed a novel catalyst 31 based on dicarboxylic acid derivative of 1,3,2-dioxaborolane for the asymmetric reduction of prochiral ketones 32. This catalyst reduces a wide variety of ketones enantioselectively in the presence of a co-reductant such as LiBH4. The mechanism involves the coordination of ketone 32 with the chiral boronate 31 and the conjugation of borohydride with carboxylic acid to furnish the chiral borohydride complex 34. Subsequent transfer of hydride from the least hindered face of the ketone yields the corresponding alcohol 35 in high ee (Scheme 3) <20060PD949>. 

REACTIVITY OF SUBSTITUENTS ATTACHED TO THE RING CARBON ATOMS AND TO THE RING NITROGEN AND SULFUR ATOMS... 

Since CHEC-II(1996) <1996CHEC-II(6)637>, there have been no developments in the chemistry concerning the reactivity of substituents attached to the ring carbon atoms of the 1,2,3-oxadiazine and 1,2,3-thiadiazine systems. 

The reactivity of substituents attached to a ring carbon atom has attracted very little attention and no new examples are reported here. 

Reactivity of the Substituent Attached to a Ring Carbon Atom. 209... 

An extensive coverage of the reactivity of substituents attached to the 1,2,5-thiadiazole ring carbon atoms exists in both CFIEC(1984) and CFIEC-II(1996). Recent developments are described in this section. 

Reactivity of Substituents Attached to Ring Carbon Atoms 5.01.7.1 Reactions of the Benzenoid Ring of Benzotriazole... 

Numerous transformations of functional groups attached to 1,2,3-oxadiazoles have been reported in the last 10 years. The reactivity of substituents attached to ring carbon atoms was thoroughly discussed in CHEC(1984) and CHEC-11(1996). 

Since the publication of CHEC-II(1996), there have been very few examples related to the reactivity of substituents attached to ring carbon atoms. One case involves the reaction of 3-benzylidene-2,3-dihydro-2-methyl-l,2-benzothiazin-4-one 1,1-dioxide 163 with the alkylidenephosphorane derived from salt 164 forming the tricyclic-fused ring compound 165 (Scheme 20) <1996J(P1)2541>. This material 165 was oxidized with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) affording the biphenyl 166. Ring-opened product 167 was produced from 165 upon exposure to />-toluene-sulfonic acid and heat. 

The reactivity of substituents attached to ring carbon atoms is of much interest, mainly when it concerns complex fused-ring thietanes linked to naturally occurring compounds. The representative example of such a case is conversion of the thymine-thietane derivative 55 to its flavin derivative 56 in the presence of 1-hydroxybenzotriazole (HOBT) and 2-(l -benzotriazole-l-yl)-l,l,3,3-tetramethyluronium tetrafluoroborate (TBTU) (Equation 12) <20050BC1937>. 

There are no significant reports updating the reactivity of substituents attached to ring carbon atoms. To explain the formation of the allylic nitro compound 44 as the major product of the nitration of methyl methacrylate 43 (Scheme 12) versus nitroacetamide 45, a plausible mechanism postulated by Murphy and co-workers... 

Considering the lack of research being performed on 1,2- and 1,3-oxathietanes and their oxidized forms, it is not surprising that there are no reports in the last decade on the reactivity of substituents attached to ring carbon atoms. 

The reactivity of substituents attached to ring carbon atoms has been covered in Section 2.22.6.4. 

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