Furans are versatile synthons in organic chemistry. Described is a general method for transforming furans into alkynes by dual C-C double-bond cleavage. The reaction is proposed to proceed by sequential [4+2] cycloaddition between furan and singlet oxygen and a formal retro-(3+2) fragmentation of the endoperoxide intermediate. A wide array of furans, including those derived from sapogenins, are amenable to this reaction, thus providing the corresponding alkynoic acids in up to 88 % yields. The synthetic utility was demonstrated by a seven-step synthesis of the proposed structure of a pregnane natural product, aglatomin B, from a known intermediate.
Reactions of 2-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)acetic acid (pypzacacH) ligand with Cu(OAc) 2 , Cu(NO 3 ) 2 , CuSO 4 , Cu(ClO 4 ) 2 or CuCl 2 produced four dinuclear Cu(II) complexes [{(MeOH)Cu(OAc)}(m-k 2 :k 1 -and one polymeric Cu(II) complex [(CuCl)(m-k 3 :k 1 -pypzacac)] n (5), respectively. The mpypzacac ligand in 3 and 4 was in situ generated via the Cu 2+ -catalyzed dehydrative esterification of acetic acid of the pypzacacH ligand. Complexes 1-5 are characterized by elemental analysis, IR and single-crystal X-ray diffraction. Complex 1 contains two {(MeOH)Cu(OAc)} fragments that are interconnected by two m-k 2 :k 1 -pypzacac À ligands, forming a dimeric structure. In 2, {Cu(pypzacac)} and {Cu(H 2 O) 2 } units are bridged by a pair of m-k 2 :k 1 -pypzacac À ligands. In 3, two {Cu(mpypzacac)} fragments are linked by two m-k 1 :k 1 -SO 4 2À ions to form a dinuclear structure. Complex 4 also adopts a dimeric structure in which {Cu(mpypzacac) 2 } and {Cu(mpypzacac)} units are interconnected by one m-k 3 :k 1 -pypzacac À ligand. Complex 5 contains a 1D chain in which (CuCl) fragments are interlinked by m-k 3 :k 1 -pypzacac À ligands. Complexes 1-5 exhibited excellent catalytic performance in the ammoxidation of alcohol to nitrile and the aerobic oxidation of alcohol to aldehyde in water. The catalytic aqueous solution was easily separated and could be reused for several cycles without any obvious decay of catalytic efficiency.
The development of new chemical transformations to simplify the synthesis of valuable building blocks is a challenging task in organic chemistry and has been the focus of considerable research effort. Here we report a chemical transformation that enables the facile and modular synthesis of synthetically challenging yet biologically important functionalized butenolides from easily accessible furans. Specifically, Diels–Alder reactions between furans and singlet oxygen generate versatile hydroperoxide intermediates, which undergo iron(II)‐mediated radical fragmentation in the presence of Cu(OAc)2 or various radical trapping reagents to afford butenolides bearing a wide variety of appended remote functional groups, including olefins, halides, azides, and aldehydes. The practical utility of this transformation is demonstrated by easy diversification of the products by means of cross‐coupling reactions and, most importantly, by its ability to simplify the syntheses of known building blocks of eight biologically active natural products.
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