Decalin ring systems

Decalin ring systems appear as structural units in a large number of naturally occurring substances, particularly the steroids. Cholic acid, for example, a steroid present in bile that promotes digestion, incorporates d5-decalin and rran -decalin units into a rather complex tetracyclic structure. 

Intramolecular Diels-Alder cyclizations of (E)- -nitro-1,7.9-decatrienes under thermal conditions and Lewis acid conditions lead to the formation of decalin ring systems with excellent endo selectivity (Eq. 8.21). This strategy is used for preparing of the AB ring system of norzoanthamine.33... 

The common structural feature of quinolizidine alkaloids is a decalin ring system with a nitrogen at one vertex. Often a second or third nitrogen atom is... 

A jS-phenethylamine moiety was incorporated into the trans-decalin ring system (45)... 

While it is not clear from these applications whether the Wharton reaction necessarily proceeds via trans diaxial epoxide opening, Ziegler et al in synthetic studies towards quassinoids, have found that there is no such stereoelectronic requirement in a rigid fraws-decalin ring system (Scheme 16). 

The oxygen bridge in oxabicyclic compounds is an electron pair donor that can stabilize a-carbocations. This characteristic renders oxabicyclic substrates more susceptible to carbocationic skeletal rearrangements resulting in the cleavage of the carbon framework. One such reaction was exploited by Sammes for the synthesis of ( )-cryptofauronol, in which treatment of 145 with Lewis acid induces rearrangement to a decalin ring system, Eq. 100 [57]. 

The feasibility of cyclizing polyenes with iminium ion initiators has received only scant attention. 72 Cyclization of imine (77) under aprotic conditions with SnCU affords predominantly rran -decalins containing endocyclic unsaturation. This mixture was deduced by H NMR analysis to contain diastereomers (78) to (81) in the indicated abundances as depicted in equation (7). The extent of asymmetric induction in forming the decalin ring system (9S,10iS 9/ ,10/ = 61 39) is significantly lower than that of related cyclizations of chiral acetal substrates. ... 

The second pathway involves the intermediacy of a carbocation derived from 2Z,6 -famesyl pyrophosphate. Following a 1,3-hydride shift, the resulting allylic carbocation can cyclize to the decalin ring system which characterises the calamenenes (76,77) and the cadinenes (79-82,85). The derivation of oplopanone (88) by ring contraction of the cadinene system seems likely. 

The related anionic oxy-Cope rearrangement of 126 to 127 was utilized for preparation of the cw-decalin ring system of superstolides A and... 

Cadinane, muurolane, and amorphane derivatives possess decalin ring system substituted with an isopropyl and two methyl groups. Of these, p-cadinene is one of the most common in essential oils. It is abundant in conifer oils. 

We have thus developed a viable entry utilizing the intramolecular Diels-Alder reaction to assemble a tricylcic ring system which can be cleaved open to the decalin ring system of the Clerodane diterpenes. The approach should also make available highly functionalized decalin derivatives which might he useful for syntheses of other natural products containing a decalin skeleton. 

Recall that the goal was to arrive at aminoketone 56. The projected Mannich reaction requires that the amino group be on the concave face surface of the m-decalin ring system. Notice that the alcohol stereochemistry in 67 is set such that an 8 2 reaction at the carbinol center would establish the required stereochemistry in 56 [We will see another approach to establishing this stereochemistry shortly]. Tosylate formation followed by acetal hydrolysis provided 68, but treatment of this material with azide failed to give any of the desired 8 2 product. Treatment of 68 with methylamine, however, gave 56 in excellent yield. Given the results with azide, it is probable that this displacement occurs with intramolecular delivery the nucleophile via involvement of an N,W-acetal (69). The final Mannich reaction proceeded as anticipated to provide luciduline (55). 

---