Of neolignans

Neolignans are also compounds composed of two phenylpropanoid units, but linked in a manner other than C8-C8 (Fig. 12.1) [10]. The compounds of this class are often chiral and naturally occurring neolignans are usually optically active, which evokes the involvement of DPs in the biosynthesis of neolignans. 

Fig. 12.10 Enantioselective formation of neolignans in Vinca rosea and Piper regnellii...

Cationic [5 + 2]cycloaddition involving a styrene and p-benzoquinone ketals in the presence of a Lewis acid is the key reaction to a number of neolignans fe nin [187]. The oxygen atoms of the dioxolane function in the cycloaddend rrinforce the a character of their neighboring carbons, and this character is transmitted to the termini which participate in the C-C bond formations. 

On the other hand, PIFA-induced oxidation of p-methoxy-substituted phenols (15) in the presence of electron-rich styrene derivatives (16) resulted in new carbon-carbon bond formation via an intermolecular 1,3-cycloaddition to afford frazzs-dihydrobenzofurans (17) stereoselectively [35,36] [Eq. (4)]. A formal synthesis of neolignans such as kadsurenone (18) and denudatin B (19) was achieved by this methodology. 

Irradiation of 2,2-dimethyl chromene through Pyrex using a 550-W Hanovia lamp initiates a retro 4 + 2 reaction to form the extended quinone methide 4, which reacts with methanol to form a pair of methyl ethers (Scheme 6A).18 Flash photolysis of coniferyl alcohol 5 generates the quinone methide 6 (Scheme 6B) by elimination of hydroxide ion from the excited-state reaction intermediate.19 The kinetics for the thermal reactions of 6 in water were characterized,20 but not the reaction products. These were assumed to be the starting alcohol 5 from 1,8-addition of water to 6 and the benzylic alcohol from 1,6-addition of water (Scheme 6). A second quinone methide has been proposed to form as a central intermediate in the biosynthesis of several neolignans,21a and chemical synthesis of neolignans has been achieved... 

Resveratrol (314) has been known to undergo hydrogen peroxide oxidation catalyzed by HRP in aqueous acetone to afford dihydrobenzofuran 315 as a main product (41%). Recently, a variety of neolignans were isolated from the Vitaceaeous plants. Of them, both... 

Lunarine (26), a constituent of Lunaria biennis M., is a member of neolignans (see Scheme 4). Biomimetic synthesis of tetrahydrolunarine (720) was performed by K3Fe(CN)e-promoted oxidation of methyl p-hydroxyphenylpropionate (721) as a key step. 721 was oxidized with K3Fe(CN)e in aq. Na2C03 (0 °C, 3.5 h) to give in 14% yield an ortho-para coupled product 722, which was further converted to the target molecule 720 (Scheme 142) . ... 

Costa, M.C.A. and Takahata, Y. (2003) Core electron binding energy (CEBE) shifts as descriptors in structure-activity relationship (SAR) analysis of neolignans tested against Loishmania donovcmi. 

Confounds 30-33 belong to a dihydrobenzofuran type of neolignan. These compounds have been obtained for the first time in the family... 

This section reviews the structural elucidation of neolignans and sesquilignans from the bark of Eucommia ulmoides. 

Table 5. The ID50 Values of Neolignans and Hydrocortisone for Inhibition of Angiogenesis and Granuloma Formation in Mouse Air Pouch...

Table 2. Structure and Occurrence of Neolignans (symbols for Ar-groups Table 3, symbols for source species Table 4)...

The racemate of conocarpin (7.2a), the simplest known member of this small group of neolignans, was again synthesized by oxidative dimerization (with FeCla) of/7-propenylphenol (76). 

The NMR spectra of neolignans pertaining to structural types 7.1, 7.7, 7.8, 8.1, 8.3, 8.4, 9.1, 9.2 and 16.2 were analyzed and their carbon shifts assigned. Clearly, CMR constitutes a powerful analytical method, of use in the recognition not only of specific neolignan types, but also of configuration and conformation 144). 

Investigations concerning the function of neolignans in plants would appear profitable in view of the insect antifeeding property of piperenone (7.9a) (J04). 

Chart 25. Schematic representation of coupling processes which lead from propenylphenol (88) and allylphenol (89) derived radicals, either by dimerization (x 2) or by addition of radical 90 [+(90)] to the indicated (in brackets) numbers of neolignans belonging to the... 

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