Bisannulation of Platinum‐Bound Isochromeno[6,7‐g]isochromene‐2,9‐Diium Derived from 3,6‐Dialkynylnaphthalen‐2,7‐Dicarboxaldehyde with Cyclohexene

Abstract

Synthesis of seven-membered carbocycles has been extensively studied especially with transition-metal catalysts. However, its application to natural products has been limited due to its property of having highly substratedependent reactivity. 1 The [4 + 3] or [5 + 2] are attractive strategy to prepare these compounds because of its efficient and rapid construction of cycloheptene bicyclic core. 2 Ortho-alkynylbenzaldehydes are frequently used as starting materials for the synthesis of a wide range of important transition-metal activated complex skeleton, which leads to a formation of isochromenylium intermediates that consequently reacts with proper dipolarophiles following the [4 + 2]-and/or [3 + 2]-cycloaddition mechanism. 3 The final product has been obtained from cycloadduct intermediate via a cascade ring-opening/protodemetalation process. 4 There are numerous synthetic applications of this process in the construction of complex bicylic products and detailed mechanistic studies have been reported on this topic. 5 Although the formation of metal-containing ylide intermediates is now well understood, whether the formation of products is by modes of [3 + 2]-or [4 + 2]-cycloaddition of a Pt-isochromenylium with alkenes are still questionable. 6 For example, the Pt-isochromenylium intermediates, generated from 2-alkynylbenzaldehydes, have underwent [4 + 2] cycloaddition with allylic alcohols as reported by Liu et al., while Iwasawa argued a selective [3 + 2] cyclization with electron-rich silyl enol ethers. 7 In the continuation of our research in [3 + 2]-platinumcatalyzed cyclization which involves the cylcloaddition of a Pt-bound isochromenylium species 1A, in situ generated from 2-alkynylbenzaldehyde 1, with an olefin to form intermediate 1B. The 1B would undergo isomerization into 1C that might form direct [4 + 2] cycloaddition from 1A with cyclohexene (Scheme 1). The result revealed that the [3 + 2] cycloaddition pathway of Pt-isochromenylium intermediates is more plausible to understand and explain the chemoselectivity of the obtained product. Thus the fastformed 1B would undergo rearrangement into the more stable 1C and then further migration into 1D by Ptinvolving process. The 1D could be tautormerized into 1E and followed by reductive elimination to afford the product 2 with reusable Pt(II). Due to the synthetic application and mechanistic interest of seven-membered ring formation, we would like to extend our process to achieve bisannulation using 3,6-dialkynylnaphthalen-2,7-dicarboxaldehyde with cyclohexene to test its feasibility. We first prepared 3,6-dialkynylnaphthalen-2,7-dicarboxaldehyde 7 as a substrate. The spacer 2,7-naphthalenedicarbaldehyde 5 was prepared in three steps following to reported literature procedures with 2,7-naphthalenediol (1) commercially available starting material: bromination, methylation, and formylation. 8 (Scheme 2) The substrate 7 was then obtained in high yield by three steps (59% over three steps) from 5 (Appendix S1, Supporting Information).Substrate 7, in h...

2,5-Diaryl 6-hydroxyphenalenones for Single-Molecule Junctions

“…In our experiment, the literature-known 2,7-dibromo-3,6dimethoxynaphthalene 8 was obtained in 78 % yield over three steps. 59,60 A Rieche formylation with dichloromethyl methyl ether in the presence of TiCl 4 in DCM afforded the formylated naphthalene derivative 9 in 74 % yield. Aldol addition of methyl tert-butylacetate to the formyl intermediate 9 using in situ prepared LDA in dry THF and subsequent reductive dehydroxylation yielded the methyl ester 11 in 86 % yield over two steps.…”