Seco skeletons

The Baldwin and Whitehead proposal [58] went on to suggest that the initial [4 + 2] adduct 119 can undergo redox exchange to give the pentacyclic intermediate 122 shown in Figure 3.6. Hydrolysis of the iminium ion functionality in 122 leads to a tetracyclic seco-skeleton containing an aldehyde functionality. The skeleton and aldehyde functionality present in the Baldwin and Whitehead tetracyclic intermediate 123 correspond exactly with the skeleton and aldehyde functional group placement in ircinals A (102) and B (103). Finally,... 

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

Condensed Bis-3-alkylpiperidines with Seco or Rearranged Skeletons. . . 226... 

Compounds from Pseudopterogorgia elisabethae can be classified according to their carbon skeletons. So far 15 carbon skeletons have been identified which are based on the serrulatane skeleton. Various cyclizations (see also Sect. 3.1, Scheme 1) of this precursor lead to newpolycycHc structures (e.g., amphilectanes, ehsabethanes, etc.) that are starting points for new degradation products (seco-, nor-, bisnor-, etc. compoimds). 

Another tetracyclic carbon skeleton, named cumbiane, has been isolated from Pseudopterogorgia elisahethae. Its representatives are the diterpenoids cumbiasin A (33) and B (34) [20] their structures and relative configurations were elucidated by interpretation of a combination of spectral data. The six-membered ring D was formed by connecting CIO and C16 of an elisabethane carbon skeleton. The carbocyclic skeleton of the cumbiasins is unprecedented and represents a new class of C20 rearranged diterpenes. The tricyclic seco-cumbiane skeleton is derived from the cumbiasins by cleavage of the C15-C16 bond. Due to intramolecular cyclizations two additional oxo-heterocycles are present in cumbiasin C (35) [20] (Fig. 7). 

Figure 1. Basic skeleton a) iridoid b) seco-iridoid (R=H or glucose)...

A fascinating contrapolarization-driven reaction is the conversion of a protoberine skeleton to an ochotensine-type alkaloid [211], The quinomethide carbon in the seco intermediate assumes a -character which is made possible by electron supply from the oxy substituent at the meta position. Note the C-N bond cleavage step entrusts this carbon an acceptor role. 

Ahmed AA, Gati T, Hussein TA, Ali AT, Tzakou OA, Couladis MA, Mabry TJ, Toth G (2003) Ligustolide A and B, Two Novel Sesquiterpenes with Rare Skeletons and Three 1,10 -seco-Guaianolide Derivatives from Achillea ligustica. Tetrahedron 59 3729... 

On the other hand, the amine moiety of daphniphylline (1) is complicated. However, in connection with the co-occurrence of seco-daphniphylline (28), whose stereostructure has also been established, the heterocyclic skeleton of the amine moiety seems to be constructed as shown in Scheme XI in which the distributions of 14C-labeled carbon atoms are based on the squalene biosynthesis from six [2-14C]MVA molecules. 

The chemical structure of vitamin D3 is closely related to its precursor, 7-dehydrocholesterol, from which it is produced by a photochemical reaction. Therefore, vitamin Do is closely related structurally to the four-ring nucleus of steroids derived from the cyclopentanoperhydro-phenanthrene ring system. No vitamin D activity is noticed until the B ring of 7-dehydrocholesterol is opened between C-9 and C-10. Thus, vitamin D3 is a 9,10-seco steroid and its carbon skeleton is numbered accordingly (Scheme I). The important aspects of its chemistry center about the 5,6,7-cis-triene structure. The formula for vitamin D3 is C27H44O and its formula weight is 384.64. 

Vakognavine is the first example of an N, C-19-seco-diterpenoid alkaloid reported and an interesting alkaloid for biogenetic speculation. The authors (116) suggested that the C-19 aldehyde may be a plausible alternate to the pseudokobusine structure as an intermediate in the biosynthesis of the modified atisine-type skeletons such as hetisine. The C-19 hydroxyl of vakognavine hydriodide (119) is reminiscent of the oxazolidine oxygen of isoatisine. 

Terms used to describe modified steroid skeletons include nor (shortening of a side chain or contraction of a ring), homo (expansion of a ring), cyclo (formation of an additional ring), seco (fission of a ring), and abeo (migration of a bond). 

A number of other synthetic approaches to the steroid skeleton are discussed in the following sections, which are devoted more specifically to the synthesis of oestrane, androstane, and pregnane derivatives, as well as to seco-compounds and steroidal alkaloids. 

The following n,m-seco-clerodanes present a diterpenoid skeleton whieh could derive from the Cm-Cn cleavage bond of a clerodane type. 

Condensed fiis-3-Alkylpiperidines with Seco or Rearranged Skeletons 321... 

CONDENSED BIS 3-ALKYLPIPERIDINES WITH SECO OR REARRANGED SKELETONS ... 

In attempts to understand the biogenesis of the prohomoery-thrinadienone of the type of compound 118, which has the same skeleton as the key intermediate 119 used in the biogenesis of the homoerythrina alkaloids, the diphenolic isoquinoline 117 was subjected to oxidation with potassium ferricyanide. However, this reaction gave unexpectedly the abnormal products phenylpropionaldehyde 120, seco-dehydrohomerythrinadienone (121), seco-homoerythrinadien-one (122), and a quinoline derivative 123 50). 

The eight new quassinoids C19 type shinjulactone B (19) with a presence of a l,2-seco-l-nor-(5->10)-a >eo-picrasan-2,5-olide skeleton have been isolated from Simaroubaceae species. 

Two new quassinoids (34) and (35) with a presence of a 1,2-seco-1 -nor-(5 - 10)-afo o-picrasan-2,5-olide skeleton were isolated from wood Eurycoma longifolia Jack [15]. 

In order to provide sufficient manoalide for continued pharmacological evaluation, F aulkner and coworkers made an extensive c ollection of L. variabilis, from different locations in Palau. From a small number of specimens of L. variabilis were isolated two new metabolites, luffariellin A (77) and Luffariella B (78) in place of manoalide and seco-manoalide [97], Despite the different carbon skeleton, the functional groups in luffariellins A and B are identical with those in manoalide and seco-manoalide, respectively, and they showed almost identical antiinflammatory properties. Both luffariellins were potent antagonists of topical induced inflammation in the mouse ear, and inhibitors of bee venom PLA2, with an IC50 value of 5.6 x 10 8 M and 6.2 x 10 8 M, for luffariellins A and B, respectively [97]. 

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