Adventures in Ring-Contraction Reactions

Abstract

Nearly two decades of work on ring-contraction reactions is discussed in this account. We have developed several reliable protocols for the rearrangement of a number of different substrates using thallium(III) salts and, more recently, hypervalent iodine(III) reagents. A variety of ring systems, such as cyclopentanes, cis-and trans-hydrindanes, and indanes, can be obtained using these methods. Additionally, the stereoselective total syntheses of the sesquiterpenes (±)-, (-)-and (+)-mutisianthol and (+)-bakkenolide A, the indole alkaloid (±)-and (+)-trans-trikentrin A, and the potential drug (±)-indatraline have been achieved through a ring-contraction approach. 1 Introduction 2 Ring-Contraction Reactions Promoted by Thallium(III) 2.1 Thallium(III)-Mediated Ring Contraction of Cyclic Ketones 2.2 Thallium(III)-Mediated Ring Contraction of Cyclic Olefins 2.3 Thallium(III)-Mediated Ring Contraction of Homoallylic Alcohols 3 Ring-Contraction Reactions Promoted by Iodine(III) 4 Conclusion Key words: total synthesis, ring contractions, oxidative rearrangements, thallium(III), hypervalent iodine O TTN (1.1 equiv) CH 2 Cl 2 , r.t., 24 h CO 2 H 87% O TTN (1.5 equiv) CH 2 Cl 2 , r.t., 48 h CO 2 H 65%

N‐Atom Deletion in Nitrogen Heterocycles

“…For N‐heterocycles used in this reaction, one N‐substituent must be benzhydryl, or both N‐substituents must bear stabilizing elements such as benzyl, allyl or be positioned α to a carbonyl group. Inspired by our previous work, [13, 16] herein we report a N‐atom deletion of N‐heterocycles via a sequence of N ‐sulfonylazidonation, Curtius‐type rearrangement and subsequent rearrangement of the 1,2‐diazene intermediate, providing a unique opportunity to promote ring contraction [17] of N‐heterocycles (Scheme 1 F).…”

N‐Atom Deletion in Nitrogen Heterocycles

“…For Nheterocycles used in this reaction, one N-substituent must be benzhydryl, or both N-substituents must bear stabilizing elements such as benzyl, allyl or be positioned a to a carbonyl group. Inspired by our previous work, [13,16] herein we report a N-atom deletion of N-heterocycles via a sequence of Nsulfonylazidonation, Curtius-type rearrangement and subsequent rearrangement of the 1,2-diazene intermediate, providing a unique opportunity to promote ring contraction [17] of Nheterocycles (Scheme 1 F).…”

Iodine(III)-Mediated Ring Contraction of 1,2-Dihydronaphthalenes: Mechanistic Insight by Computational Investigations

“…One of them is the ring contraction of 1,2‐dihydronaphthalenes promoted by thallium(III) or by iodine(III). This oxidative rearrangement has the main feature to provide trans ‐1,3‐disubstituted indanes in a diastereoselective manner, constituting the key step in the chemical synthesis of (±)‐indatraline, as well as in the total synthesis of natural products, such as (+)‐mutisianthol, and (+)‐ trans ‐trikentrin (Scheme ) . A mechanism has been suggested to explain the diastereaselectivity of this transformation .…”