New types of cyclic diazo compounds capable of Rh(II)-catalyzed spirocyclizations with tetrahydrofuran have been discovered. The formation of the spirocyclic framework is thought to proceed via the formation of Rh(II) carbene species followed by interaction with the Lewis basic oxygen atom of tetrahydrofuran to give oxonium ylide species. The latter evolves predominantly via the Stevens type rearrangement leading to an [n + 1] ring expansion of the tetrahydrofuran moiety, which results in the formation of a medicinally relevant 6-oxa-2-azaspiro[4.5]decane scaffold. The spirocyclization process was often observed in competition with mechanistically distinct C−H insertion into a tetrahydrofuran molecule. This competing process gave compounds based on the 3-(tetrahydrofur-2-yl)pyrrolidine scaffold, which are also relevant from the medicinal chemistry standpoint. These findings enrich the available arsenal of metal-catalyzed spirocyclization methods based on the use of cyclic diazo compounds.
Attempted use of the recently described (E)-3-arylidene-4-diazopyrrolidine-2,5-diones in the Rh(II)-catalyzed condensation with nitriles to form 1,3-oxazoles led to an unexpected outcome. The nitrilium ylide species thought to form on Rh(II)-catalyzed...
This review summarizes recent developments in multicomponent reactions of diazo compounds. The role of diazo reagent and the type of interaction between components was analyzed to structure the discussion. In contrast to previous reviews on related topics mostly focused on metal catalyzed transformations, a substantial amount of organocatalytic or catalyst-free methodologies is covered in this work.
We report on a facile method for
the preparation of 2-benzoxepine
derivatives as a result of Rh(II)-catalyzed decomposition of diazo
arylidene succinimides in the presence of aldehydes. The process is
thought to involve the formation of styryl carbonyl ylide which undergoes
1,7-electrocyclization and subsequent 1,5-hydrogen shift. In some
cases, the competition of the target reaction and [3+2] dipolar cycloaddition
of the intermediate carbonyl ylide to another molecule of diazo substrate
was observed. Generally, the desired 2-benzoxepines were isolated
in good to high yields and high diastereoselectivity. The developed
original approach toward a 2-benzoxepine core via formal [5+2] cycloaddition
of styryl carbenoids and aldehydes significantly expands the arsenal
of synthetic methods for producing this scaffold.
1-Nicotinoylbenzotriazole (NicBt) was uncovered as an efficient protecting agent for the site-selective acylation of resorcinol-type phenolic groups with almost equal reactivity. The use of NicBt allows selective protection of the 7-OH group in 5,7-dihydroxycoumarins in one simple scalable step, while combination of the nicotinoylation with tosylation–denicotinoylation or silylation–denicotinoylation yields 5-OH-protected 5,7-dihydroxycoumarins. Furthermore, nicotinoylated 5,7-dihydroxycoumarins proved useful in a gram-scale three-step preparation of a 2,2-dimethylpyrano[2,3-f]coumarin, a key intermediate for the synthesis of calanolide A, an HIV reverse transcriptase and Mycobacterium tuberculosis inhibitor, and its active analogues.
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