Radicals in organic synthesis: part 2

Cyclization reactions of bis(allenes) for the synthesis of polycarbo(hetero)cycles

“…On the other hand, the regioselectivity has been controlled by the 50 nature of the substituents attached to the allene moiety; however, in some cases it has been modulated by the reaction conditions. By resemblance with both enallenes and ynallenes, when we raise the study of the intramolecular [2+2] cycloaddition of bis(allenes), is essential to consider the number of possible 55 regioisomers, depending their formation of the π-component involved in the process. Then, if we use a symmetric bis(allene), four possible regioisomers can be formed, head-to-head, tail-totail, head-to tail and tail-to-head adducts (Scheme 1).…”

“…Interestingly, treatment of compound 8 under thermal conditions gave [3,3]-sigmatropic rearrangement product 9 as major product and the desired [2+2] cycloadduct 10 in low yield. 13 After testing 55 different reaction conditions using Mo(CO) 6 , PdCl 2 (PPh 3 ) 2 and Pd(PPh 3 ) 4 as transition metal catalysts, the authors found that the combination of CuBr 2 and iPr 2 NH afforded the corresponding…”

“…39 The applicability of the multicomponent reactions has been widely demonstrated in the synthesis of natural products, 40 and medicinal chemistry. 41 It has been reported that palladium-catalyzed coupling reaction 55 of propargylic carbonates with boronic acids or organozinc reagents afford alkynes or allenes depending on the steric hindrance of the starting materials. 42 Taking into account this methodology, recently, Ma and col. have reported the palladium (0)-catalyzed three component coupling reaction of 60 1,5-bis(allenes), propargylic carbonates and organoboronic acids.…”

“…The potential and utility of radical reactions in the construction of carbocyclic and heterocyclic compounds has been widely demonstrated. 55 Concerning the reactivity of allene derivatives, for long time, this peculiar functionality have been discriminate 45 against radical processes due to the lack of chemo-, regio-, and stereoselectivity. However, in recent years, interest in radicalbased transformations of allenes has been revitalized due to the accomplishment of the synthesis of target molecules and the results obtained from theoretical investigations.…”

“…56 The efficiency of this transformation depends on the chain length separating both reactive entities and is the major factor for regiocontrol. Thus, radical cyclization can takes place on the central carbon atom 55 via the dig mode of ring closure or to the internal carbon atom via the trig mode of ring closure. Reaction of bis(allenes) 1 with p-tosyl bromide or ptosylseleniumbromide in the presence of a catalytic amount of AIBN afforded trans-fused cyclopentane compounds 161, 60 incorporating vinyl sulfones, vinyl bromides or selenophenyl functionalities in their structure (Scheme 58).…”