Polycyclization cationic

Scheme 1.39. Synthesis of an indole derivative by a three fold cationic polycyclization.

Some excellent examples of cationic polycyclizations, especially in the field of steroid synthesis, were described in Chapter 1. However, these polycyclizations can also be performed using a radical as initiator. Such reactions can be divided into those based on serial 6-mdo-trig cyclizations from polyene acyl precursors [92], radi-... 

Deuterium secondary isotope effects in an olefinic cationic polycyclization have been reviewed by Borcic and coworkers237. 

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

Cationic polar cycloaddition, 16, 289 (19, xi) Cations, polycyclic aromatic nitrogen, 55,261 Chalcogen heterocyclic chemistry, some recent developments in, 71, 115 Chemistry... 

Cations, polycyclic aromatic nitrogen (review), 55, 261 Cesium fluoroxysulfate fluorination of... 

SCHEME 10.3. Enantioselective thiourea-catalyzed cationic polycyclization. 

In 2014, Canesi s group also reported on a remarkable application of their chemistry that enabled them to develop an asymmetric synthesis of the tetracyclic main core of kaurane diterpenes [131]. The phenolic non-conjugated enyne 249 was elaborated to fulfill the reactivity and stereochemistry requirements of the intended oxidative cationic polycyclization [132], in tandem with apinacolic transposition. After chlorination of the benzylic alcohol of 249 with inversion of configuration, the use of BTI rapidly promoted this dearomative tandem process, which thus... 

Oomens J, Tielens AGGM, Sartakov BG, Von Helden G, Meijer G (2003) Laboratory infrared spectroscopy of cationic polycyclic aromatic hydrocarbon molecules. Astrophys J 91 968-985... 

Also, the cationic polycyclization of polyenes is possible with designed thioureas bearing an additional aromatic moiety in order to stabilize some interesting cation-jt interactions for enantioselectivity induction next to the traditional anion binding (Scheme 7.37) [55]. 

Polycyclic terpenoids are metabolite products that are commonly found in nature and have a variety of biological activities. According to the Stoik-Eschenmoser hypothesis, many polycyclic isoprenoids are biosynthesized from simple linear polyprenoids such as geraniol, famesol, geranylgeraniol, and squalene via cationic polycyclization, which is considered to one of the most complex carbon-carbon bond-forming reactions in nature [1]. The complicated polycyclic structures of isoprenoids, which have many chiral centers including quaternary carbons, are stere-oselectively constructed by cyclases in an impressive key step (Schemes 9.2 and 9.3) [2],... 

Furthermore, Jacobsen s group reported the development of a new thiourea catalyst for the enantioselective bicycliza-tion of hydroxylactams [54]. The enantioselective cationic polycyclization reactions catalyzed by bifunctional thiourea derivatives appear to engage stabilizing jt-cadon interactions as a principal element of enantioselectivity (Scheme 9.41). 

Serratenediol, a pentacyclic triteipenoid containing a unique seven-membered central ring has been concisely synthesized using double epoxide-initiated stereoselective cationic polycyclizations mediated by MeAlClj as key steps (Scheme 9.44) [17h]. (+)-a-Onocerin has been also synthesized in four steps via double epoxide-initiated stereoselective polycychzations (Schane 9.44) [17i]. 

Inspired by the previous work, the Jacobsen group further developed the enan-tioselective cationic polycyclization. After careful studies on the reaction conditions, the authors selected chiral thiourea catalyst 98 for the asymmetric cyclization of hydroxylactams 99, affording the products 100 in good yields and with excellent enantioselectivity (Scheme 36.26) [32]. 

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