Gem-Methyls

NMR spectroscopy is a convenient method for structural study of the equilibrium between the colored and the colorless form of spirobenzopyran. In the H-NMR spectra, the chemical shifts of gem methyl groups in 3 -position, (V-methyl group,2,11 and methine protons in 3- and 4-position are important to distinguish between the colored and colorless forms. 

Figure 16.5 A catch-and-release ICAT design incorporates a gem-methyl group and an isopropyl group on either side of a disulfide bond within its spacer arm. The hindered disulfide permits the use of standard reducing gel electrophoresis conditions using DTT without reduction. After purification on a (strept)avidin affinity column, however, the disulfide group can be cleaved with TCEP, which provides recovery of the labeled peptides prior to mass spec separation.

We were able to substitute the gem-methyls in the ketal ring with gem-phenyls. Compound 23 shows that this perturbation was quite harmful to both Reactions I and V. This result was unexpected since X-ray structure showed the gem-methyls to be well removed from the site of action (8) and one would expect gem-phenyls to be situated similarly. 

Furantriol (18.27), isolated from L. mitissimus (84), is one of the few lactarane sesquiterpenes in which one of the gem-methyl groups at C-11 is oxidized and it was chemically correlated (82) with lactarorufin B (11.71), another example of this kind. The Polish authors suggested that lactone 11.71 was enzymatically formed from furan 18.27, and that a C-15 oxidized sesquiterpene of the velutinal type was the common precursor of both compounds in the mushroom (84). Actually, the possibility for the C-15 methyl group to be oxidized at an early stage of the lactarane biosynthesis seems to be confirmed by the recent finding of C-15 hydroxylated protoilludane sesquiterpenoids (5.1 and 5.2) in L. violascens (23) (Table 5). 

French workers have extensively explored type I intramolecular ene reactions of allenic aldehydes which give dienols. Thermolysis of aldehydes and ketones such as (158) affords ene adducts (159 42%, 150 C, 30 min), (160 75%, 150 "C, 60 min), (161 70%, 200 C, 15 min), (162 52%, 200 C, 60 min), (163 65%, 230 °C, 60 min) and (164). Lewis acid catalysis was investigated with limited success (Scheme 25). Comparison of yields and reaction times indicates that buttressing gem methyl groups and trisubstituted allenes facilitate the reaction, while use of ketones and formation of six- rather than five-membered rings retard it. 

In plukenetiones A-C and sampsoniones A-J the benzoyl moiety was vicinal to the aliphatic quaternary carbon that supports gem-methyls. This structural characteristic could be considered in order to extend the... 

Fig. 2.18 The splitting of the gem-methyl symmetric bending modes is inversely dependent on the CH3—C CH3 bond angle.

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