The Latin rectus Does Not Mean “Right” (in the Relative-Direction Sense): On the R/S System of Absolute Configuration

Terminology

(+)/(−)- and d/l–

In 1811, the French scientist François Arago discovered that slices of quartz cut perpendicular to the optical axis exhibit optical rotation (Arago, 1811). In 1819, Jean-Baptiste Biot discovered that optical rotation can be dextrorotatory or levorotatory, and that aqueous tartaric acid is optically active.

When looking toward the light source:

• If the plane of polarization is rotated to the right (clockwise), the substance is “dextrorotatory.” In French, dextrogyre; in Japanese, 右旋性. Formerly indicated by a lowercase italic d- before the compound name, now written as (+)- (e.g., (+)-carvone).
• If the plane rotates to the left (counterclockwise), the substance is “laevorotatory or levorotatory.” In French, lévogyre; in Japanese, 左旋性. Formerly written with l-; now written as (−)- (e.g., (−)-carvone).

D/L

Independently of optical rotation, the Fischer–Rosanoff convention—D/L notation— distinguishes enantiomers of sugars and amino acids. It is based on D-(+)-glyceraldehyde and L-(−)-glyceraldehyde. Fischer arbitrarily assigned the absolute configurations to the two enantiomers of glyceraldehyde and named the one whose OH at C-2 appears on the right side in a Fischer projection the D-form (from dexter “right”), and the one on the left the L-form (from laevus “left”). Later, when the absolute configurations of the glyceraldehydes were determined, the assignment D = (+) turned out to be correct by chance.

D and L are often written in small capitals.

Dextro- and laevo- appear elsewhere:

D-(+)-glucose (grape sugar; in Japanese, ブドウ糖) used to be called dextrose (in Japanese, 右旋糖), and D-(−)-fructose was called laevulose/levulose (in Japanese, 左旋糖). The name levulose was based on levorotation, which caused confusion because fructose belongs to the D-series. The modern name fructose comes from Latin fructus “fruit” + -ose.

Other examples include dextroamphetamine ((+)-amphetamine), levoamphetamine ((−)-amphetamine), dextrocardia (in Japanese, 右胸心), and levocardia (in Japanese, 左胸心).

R/S

Absolute configuration is specified using the Cahn–Ingold–Prelog (CIP) priority rules, which also define E/Z notation and others. For a stereocenter with four substituents, the lowest-priority group is oriented away from the viewer. If tracing the remaining substituents in order of priority gives a clockwise path, the configuration is R; if it gives a counterclockwise path, the configuration is S.

Thus, D-(+)-glucose is (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal, and L-(−)-glucose is the enantiomer (2S,3R,4S,5S)-….

Examples of names incorporating R/S include arketamine ((R)-(−)-ketamine) and esketamine ((S)-(+)-ketamine).

Etymology

Now to the main topic. The prefixes of the D/L system (and Δ/Λ for octahedral complexes) derive from dextro- from dexter (adj.) “right; right-hand side,” and laevo- from laevus (adj.) “left; left-hand side.” Rosanoff initially used the Greek letters δ/λ– to avoid confusion with Fischer’s d/l– series. In his 1906 JACS paper explains, he writes:

Thus, ordinary glucose and its corresponding fructose (levulose) are designated, respectively, d-glucoase and d-fructose, not withstanding the levo-rotation of the latter.

The substances so correlated include, as yet, only the monosaccharides and their derivatives. In other cases the symbols d and l denote dextro- and levo-rotation. The reform proposed in the following pages, and the importance of avoiding possible confusion, will justify my employing in this paper the Greek δ and λ as family-symbols in place of the customary but misleading d and l.

— Rosanoff, M. A. On Fischer’s classification of stereo-isomers. J. Am. Chem. Soc. 1906, 28, 114.

• dexter (Latin adj.): meanings 1) right (relative direction), right-hand (antonym: laevus, scaevus, sinister); 2) skillful; 3) fortunate, favorable; 4) proper, fitting
• laevus (Latin adj.): meanings 1) left, on the left side; 2) (figuratively) clumsy, awkward; 3) (figuratively) foolish; 4) unlucky

Then, textbooks state that R and S derive from Latin rectus and sinister.

Cahn, Ingold, and Prelog (1956) wrote:

The suggested indications for asymmetry leading, under the sequence and conversion rules, to a right- and left-handed pattern, are capital italic R and S, respectively, where R derives from the Latin rectus, meaning “right”, and S from sinister. This last word means “left” not only in Latin: it appears with that significance in several modern languages: and, although in English its common meaning is the symbolic derivative, “of evil omen”, in a few special uses it retains its original meaning of “left”.

— Cahn, R. S.; Ingold, C. K.; Prolog, V. The specification of asymmetric configuration in organic chemistry. Experientia 1956, 12, 81.

• rectus (perfect passive participle of regō “to keep or lead straight, to guide”): meanings 1) led straight along, drawn in a straight line, straight, upright; 2) (in general) right, correct, proper, appropriate, befitting; 3) (in particular) morally right, correct, lawful, just, virtuous, noble, good, proper, honest.
• sinister (Latin adj.): meanings 1) left; 2) perverse, bad; or adverse, hostile; 3) (religion) auspicious (for Romans) or inauspicious (for Greeks).

Rectus does not have the directional sense “right.” Why do Japanese organic chemistry textbooks say “rectus means ‘右（right (direction)）’ in Latin”? Because English right—the obvious translation of rectus—is polysemous: “straight,” “correct,” and also “right (direction).”

How Japanese Textbooks Describe “Rectus”

Bruce Organic Chemistry (7th ed.) states:

If the arrow points clockwise, the compound has the R configuration (R is for rectus, which is Latin for “right”).