Use of Δ (delta) to describe alkenes

Question

In a lot of (mainly older) papers discussing organic synthesis, Δ is used to describe alkenes. As far as I know, this is no longer (if ever) IUPAC recommended nomenclature, so I haven't been able to find any reference to its intended use.

The main area I'd seen it used was in steroid synthesis, where I assumed it had some specific use relating to that class of natural product (i.e. related to the biosynthesis etc.). However it does appear in other places, such as in the synthesis of Rifamycin:

_{Scheme from Nicolau Rapamycin synthesis. Taken from Classics in Total Synthesis I}

"… the Δ^35,36 double bond …" is used in the text to describe the alkene that gets reduced. Given that this could easily be referred to as the (E)-alkene or even just 206, I suspect that there is a special reason why Nicolau chose Δ^35,36 however that reason isn't apparent to me.

Answer 1

Steroid nomenclature

The Δ notation was obviously in use in some fields of the chemistry of natural compounds and/or by some publishers, but probably never recommended by IUPAC. A mention in older specialized IUPAC rules is e.g. in IUPAC-IUB revised tentative rules for nomenclature of steroids (1969):

Rule 2S-4 (extended version of Rule S-4)
…
I. Unsaturation is indicated by changing terminal ‘-ane’ to ‘ene’, ‘-adiene’, ‘-yne’ etc., or ‘-an’ to ‘-en’, ‘-adien’, ‘-yn’ etc.; e.g. 5β-cholest-6-ene, 5β-cholesta-7,9(11)-diene, 5-spirosten; see also the names of examples (22)-(25). [For uniformity with the IUPAC Rules cited above, the conventions of Chemical Abstracts are used also in the present Rules for the position of locants (positional numerals) and designation of unsaturation. In such matters, and in use of Δ (Greek capital delta) to designate unsaturation (which is not recommended by IUPAC), authors should respect the house customs of the journals to which their papers are submitted.]

Later it became just a footnote in (currently obsolete) IUPAC Definitive rules for nomenclature of steroids (1971/1972) (p. 303).

Pretty recent IUPAC Nomenclature of steroids (Recommendations 1989) states:

3S-2.5. Unsaturation
…
3) The use of Δ (Greek capital delta) is not recommended to designate unsaturation in individual names. It may be used, however, in generic terms, like 'Δ⁵-steroids'.

General natural product nomenclature

However, presently, the most widely known use of this delta notation is not in steroids, but in cannabinoids (to specify positional isomers of unsaturated compounds):

Δ⁹-tetrahydrocannabinol (Δ⁹-THC, THC)

Δ⁸-THC

(Please note that there are at least two different numberings for cannabinoids, so you might meet, especially in older literature, names like Δ¹-THC and Δ¹⁽⁶⁾-THC instead of Δ⁹-THC, resp. Δ⁸-THC, but this is a different issue.)

Recent organic chemistry nomenclature

In the recent Nomenclature of Organic chemistry by IUPAC, Recommendations and preferred names (2013), besides the non-preferred notation for double bond junctions already mentioned in another answer, the delta notation is allowed even for the preferred names (PINs):

P-25.7.1.2 Localized double bonds

If it is necessary to identify isomers that differ only by virtue of the location of localized double bonds, this differentiation is indicated by the use of the Greek letter Δ. The locant cited corresponds to the first cited locant of the localized double bond.

Examples:

1,6-dimethyl-Δ¹-heptalene (PIN)

1,6-dimethyl-Δ^1(10a)-heptalene (PIN)

(Howewer these won't occur very often, the structure in the examples above looks like conjugated system, but it's non-aromatic).

Comma usage

Also note that the coma was/is being used for multiple double bond position specification, as seen in excerpt from Fieser: Natural Products Related To Phenanthrene (1949)

Examples. Δ^7,14-Cholestadiene (…), Δ^7,9(11)-cholestadiene (…), Δ⁸⁽¹⁴⁾-cholestenol-3β

(But note that this comma usage somewhat collides with ring assemblies notation mentioned in another answer.)

In your example, the comma usage is rather incorrect, should be Δ³⁵⁽³⁶⁾ (so called compound locant, see e.g. A: Numbering bonds in cyclic compounds), instead of Δ^35,36. And here the locants differ by 1, so Δ³⁵ should be sufficient.

Answer 2

@Loong's encyclopaedic knowledge of IUPAC nomenclature prompted me to have another look through the (irritatingly unsearchable) 2013 recommendations and the 2004 draft upon which they were built.

The only use of the uppercase Δ listed is when describing a double bond that links two cyclic systems:

P.28.2.2: Ring assemblies with a double bond junction

When two cyclic systems are linked by a double bond [...] the presence of a double bond is indicated by the Greek letter Δ, and the point of attachment of the ring is given by superscript locant numbers. This method is not continued in these recommendations

The example given is 1'1-bi(cyclopentylidene) (PIN) which should be used in preference of Δ^1,1'-bicyclopentylidene.

Based on this, the example given in the question, using Δ to describe an unsaturated carbonyl would appear invalid, and used for convenience rather than to convey anything specific.