Construction of Complex Polycyclic Systems using Gold Catalyzed Intramolecular Diyne/Enyne/ Hydroalkoxylation Reactions

Abstract

First section of chapter 1 deals with gold catalyzed synthesis of ring fused furo[3,2,b]pyrans and furo[3,2,b]furans. Furo[3,2,b]pyrans and furo[3,2,b]furans are ubiquitous structural segments found in a number of natural products including polyether containing marine toxins. Synthesis of furo[3,2,b]pyrans 2a was accomplished from the bis-propargyl ethers 1a, while the synthesis of furo[3,2,b]furans 2b was accomplished from the prenyl propargyl ethers 1b. Scheme 1: Synthesis of furo[3,2,b]pyrans and furo[3,2,b]furans Second section of chapter 1 describes an unusual ring-contraction rearrangement route to functionalized 2,8-oxymethano-bridged di and triquinane. During the course of investigations concerning the total synthesis of 6-oxabicyclo[3.2.1]octane framework containing natural products, an unusual ring-contraction rearrangement sequence was observed in the reaction of 5-substituted 1-methyl-4-isopropenyl-6-oxabicyclo[3.2.1]octan-8-ones 4 to the 2,8-oxymethano-bridged diquinanes 5. The reaction was further demonstrated in the synthesis of triquinane 7 framework. Scheme 2: Synthesis of functionalized di and triquinane In third section of chapter 1 gold catalyzed synthesis of isochromanones and isoquinolones from suitable substituted allyl propargyl ethers was discussed. Synthesis of isochromanones and isoquinolones comprising a quaternary center with high diastereoselectivity was realized via AuCl3 catalyzed tandem intramolecular exo-dig heterocyclization/enol isomerization/Claisen rearrangement sequence in excellent yields. The reaction was general and amenable for the synthesis of structurally diverse analogues. Scheme 3: Synthesis of isochromanones and isoquinolones Forth section of chapter 1 consists of gold catalyzed intramolecular hydroalkoxylation assisted ring opening of furans to the corresponding saturated -keto esters. During the course investigations concerning gold catalyzed intramolecular enyne cyclization reactions, an interesting ring opening of furans in furyl propargyl alcohols to the corresponding tetrahydrofuran appended saturated -keto esters exclusively driven by intramolecular hydroalkoxylation of the alkyne was observed. Reaction of furyl propargyl alcohols without free hydroxyl group, under similar conditions afforded the conjugated enynes involving dehydration/ketalization. Scheme 4: Synthesis of saturated -keto esters and enynes Chapter 2 delineates the enantiospecific synthesis of bicyclo[4.2.2]decadienes 15 via gold catalyzed tandem enyne cyclization, semipinacol rearrangement reaction. Bicyclodecadienes are key structural units of natural products nakafuran-8 and pallescensin B. Scheme 5: Synthesis of bicyclo[4.2.2]decadienes