Bargellini condensation of coumarins. Expeditious route to o-carboxyvinylphenoxyisobutyric acids and application to the synthesis of sesquiterpenes helianane, heliannuol A and heliannuol C

Meta-analysis of defunctioning stomas in low anterior resection for rectal cancer Br J Surg (Br J Surg 2009; 96: 462–472)

“…The envisioned retrosynthetic analysis of ( S )‐7,11‐helianane ( 1 ) and ( S )‐5‐chloro‐7,11‐helianane ( 3 ) starts with the well‐documented (in its racemic form)5b,5c dialdehyde precursor 4 and entails Trost's palladium‐catalyzed asymmetric allylic alkylation (Pd‐AAA)8,9 on meta ‐cresol, followed by a stereoselective aromatic Claisen rearrangement as the key steps to install the required stereogenic center (Scheme ). Robust literature precedents, based upon the concept of dynamic kinetic asymmetric transformation (DYKAT),8,9 support the feasibility of the planned asymmetric alkylation of the starting phenol 10.…”

“…Failure to synthesize the benzooxocene intermediate 13 through McMurry coupling prompted us to retrace the final synthetic steps reported by Venkateswaran et al5b,5c to access ( S )‐7,11‐helianane ( 1 ). Accordingly, double Wittig olefination of dialdehyde 4 would set the stage for the subsequent ring‐closing metathesis, delivering 13 .…”

“…Accordingly, double Wittig olefination of dialdehyde 4 would set the stage for the subsequent ring‐closing metathesis, delivering 13 . Unexpectedly, attempts to reproduce the Wittig olefination5b,5c proved to be exceedingly troublesome, affording the desired alkene 15 in an unacceptable 17 % yield at best. Similar frustrating outcomes were encountered by employing commercially available Tebbe reagent.…”

“…The first total synthesis of racemic 7,11‐helianane ( 1 ) was reported by Snieckus and Stefanovic in 1998 6. Later, the group of Venkateswaran entered the field by publishing three communications5a,5b,5e and two full papers5c,5d on the subject. Remarkably, in their last communication, Venkateswaran et al5e also accomplished the first synthesis of (±)‐5‐bromo‐ ( 2 ) and (±)‐5‐chloro‐7,11‐helianane ( 3 ).…”

Biomimetic diversity-oriented synthesis of benzannulated medium rings via ring expansion

“…For example, although ring-closing metathesis was used in the total synthesis of aspercyclide C, it proved ineffective for accessing the closely related aspercyclides A and B 19,20 . Variable, substrate-dependent yields (30–94%) have been reported in ring-closing metathesis approaches to heliannuol A and related helianane congeners 21,22,23,24,25,26 . In the sphere of library synthesis, elegant cyclization-based approaches to medium rings have been reported 27,28,29,30,31 , but are generally limited in their ability to accommodate diverse backbone substituents, ring sizes, and other structural factors that affect cyclization efficiency.…”